Math 140 by Paul... · Math 140 © 2007 Paul Dawkins 3 Integrals Introduction In this chapter we...

Math 140

Paul Dawkins

Math 140

© 2007 Paul Dawkins i http://tutorial.math.lamar.edu/terms.aspx

Table of Contents Integrals .......................................................................................................................................... 3

Introduction ................................................................................................................................................ 3 Indefinite Integrals ..................................................................................................................................... 5 Computing Indefinite Integrals ................................................................................................................ 12 Substitution Rule for Indefinite Integrals ................................................................................................. 22 More Substitution Rule ............................................................................................................................ 35 Area Problem ........................................................................................................................................... 48 The Definition of the Definite Integral ..................................................................................................... 58 Computing Definite Integrals ................................................................................................................... 68 Substitution Rule for Definite Integrals ................................................................................................... 80

Applications of Integrals ............................................................................................................. 91 Introduction .............................................................................................................................................. 91 Average Function Value ........................................................................................................................... 92 Area Between Curves ............................................................................................................................... 95 Volumes of Solids of Revolution / Method of Rings ............................................................................. 106 Volumes of Solids of Revolution / Method of Cylinders ....................................................................... 116 More Volume Problems ......................................................................................................................... 124

Extras .......................................................................................................................................... 135 Introduction ............................................................................................................................................ 135 Proof of Various Integral Facts/Formulas/Properties ............................................................................. 136 Area and Volume Formulas ................................................................................................................... 148 Types of Infinity ..................................................................................................................................... 152 Summation Notation .............................................................................................................................. 156 Constants of Integration ......................................................................................................................... 158

Integration Techniques .............................................................................................................. 163 Introduction ............................................................................................................................................ 163 Integration by Parts ................................................................................................................................ 165 Integrals Involving Trig Functions ......................................................................................................... 176 Trig Substitutions ................................................................................................................................... 187 Partial Fractions ..................................................................................................................................... 199 Integrals Involving Roots ....................................................................................................................... 208 Integrals Involving Quadratics ............................................................................................................... 210 Using Integral Tables ............................................................................................................................. 219 Integration Strategy ................................................................................................................................ 223 Improper Integrals .................................................................................................................................. 230 Comparison Test for Improper Integrals ................................................................................................ 238

Polar Coordinates ...................................................................................................................... 245 Introduction ............................................................................................................................................ 245 Polar Coordinates ................................................................................................................................... 246 Tangents with Polar Coordinates ........................................................................................................... 257

Sequences and Series ................................................................................................................. 260 Introduction ............................................................................................................................................ 260 Sequences ............................................................................................................................................... 261 More on Sequences ................................................................................................................................ 272 Series – The Basics ................................................................................................................................ 278 Series – Convergence/Divergence .......................................................................................................... 285 Series – Special Series............................................................................................................................ 294

Vectors ........................................................................................................................................ 303 Introduction ............................................................................................................................................ 303 Vectors – The Basics .............................................................................................................................. 304 Vector Arithmetic ................................................................................................................................... 309 Dot Product ............................................................................................................................................ 315 Cross Product ......................................................................................................................................... 324

http://tutorial.math.lamar.edu/terms.aspx

Math 140

© 2007 Paul Dawkins ii http://tutorial.math.lamar.edu/terms.aspx

Three Dimensional Space .......................................................................................................... 330 Introduction ............................................................................................................................................ 330 The 3-D Coordinate System ................................................................................................................... 331 Equations of Lines .................................................................................................................................. 338 Equations of Planes ................................................................................................................................ 345 Quadric Surfaces .................................................................................................................................... 348


Math 140

© 2007 Paul Dawkins 3 http://tutorial.math.lamar.edu/terms.aspx

Integrals

Introduction In this chapter we will be looking at integrals. Integrals are the third and final major topic that will be covered in this class. As with derivatives this chapter will be devoted almost exclusively to finding and computing integrals. Applications will be given in the following chapter. There are really two types of integrals that we’ll be looking at in this chapter : Indefinite Integrals and Definite Integrals. The first half of this chapter is devoted to indefinite integrals and the last half is devoted to definite integrals. As we will see in the last half of the chapter if we don’t know indefinite integrals we will not be able to do definite integrals. Here is a quick listing of the material that is in this chapter. Indefinite Integrals – In this section we will start with the definition of indefinite integral. This section will be devoted mostly to the definition and properties of indefinite integrals. Computing Indefinite Integrals – In this section we will compute some indefinite integrals and take a look at a quick application of indefinite integrals. Substitution Rule for Indefinite Integrals – Here we will look at the Substitution Rule as it applies to indefinite integrals. Many of the integrals that we’ll be doing later on in the course and in later courses will require use of the substitution rule. More Substitution Rule – Even more substitution rule problems. Area Problem – In this section we start off with the motivation for definite integrals and give one of the interpretations of definite integrals. Definition of the Definite Integral – We will formally define the definite integral in this section and give many of its properties. We will also take a look at the first part of the Fundamental Theorem of Calculus. Computing Definite Integrals – We will take a look at the second part of the Fundamental Theorem of Calculus in this section and start to compute definite integrals. Substitution Rule for Definite Integrals – In this section we will revisit the substitution rule as it applies to definite integrals.


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Math 140


Indefinite Integrals

In the past two chapters we’ve been given a function, ( )f x , and asking what the derivative of

this function was. Starting with this section we are not going to turn things around. We now want to ask what function we differentiated to get the function ( )f x .

Let’s take a quick look at an example to get us started. Example 1 What function did we differentiate to get the following function. ( ) 4 3 9f x x x= + − Solution Let’s actually start by getting the derivative of this function to help us see how we’re going to have to approach this problem. The derivative of this function is, ( ) 34 3f x x′ = + The point of this was to remind us of how differentiation works. When differentiating powers of x we multiply the term by the original exponent and then drop the exponent by one. Now, let’s go back and work the problem. In fact let’s just start with the first term. We got x4 by differentiating a function and since we drop the exponent by one it looks like we must have differentiated x5. However, if we had differentiated x5 we would have 5x4 and we don’t have a 5 in front our first term, so the 5 needs to cancel out after we’ve differentiated. It looks then like

we would have to differentiate 515

x in order to get x4.

Likewise for the second term, in order to get 3x after differentiating we would have to

differentiate 232

x . Again, the fraction is there to cancel out the 2 we pick up in the

differentiation. The third term is just a constant and we know that if we differentiate x we get 1. So, it looks like we had to differentiate -9x to get the last term. Putting all of this together gives the following function,

( ) 5 21 3 95 2

F x x x x= + −

Our answer is easy enough to check. Simply differentiate ( )F x .

( ) ( )4 3 9F x x x f x′ = + − =


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So, it looks like we got the correct function. Or did we? We know that the derivative of a constant is zero and so any of the following will also give ( )f x upon differentiating.

( )

( )

( )

5 2

5 2

5 2

1 3 9 105 21 3 9 19545 21 3 346995 2 123

.

F x x x x

F x x x x

F x x x x

etc

= + − +

= + − −

= + − +

In fact, any function of the form,

( ) 5 21 3 9 , is a constant5 2

F x x x x c c= + − +

will give ( )f x upon differentiating.

There were two points to this last example. The first point was to get you thinking about how to do these problems. It is important initially to remember that we are really just asking what we differentiated to get the given function. The other point is to recognize that there are actually an infinite number of functions that we could use and they will all differ by a constant. Now that we’ve worked an example let’s get some of the definitions and terminology out of the way. Definitions Given a function, ( )f x , an anti-derivative of ( )f x is any function ( )F x such that

( ) ( )F x f x′ = If ( )F x is any anti-derivative of ( )f x then the most general anti-derivative of ( )f x is called

an indefinite integral and denoted,

( ) ( ) , is any constantf x dx F x c c= +∫

In this definition the ∫ is called the integral symbol, ( )f x is called the integrand, x is called

the integration variable and the “c” is called the constant of integration.


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Note that often we will just say integral instead of indefinite integral (or definite integral for that matter when we get to those). It will be clear from the context of the problem that we are talking about an indefinite integral (or definite integral). The process of finding the indefinite integral is called integration or integrating f(x). If we need to be specific about the integration variable we will say that we are integrating f(x) with respect to x. Let’s rework the first problem in light of the new terminology. Example 2 Evaluate the following indefinite integral.

4 3 9x x dx+ −∫ Solution Since this is really asking for the most general anti-derivative we just need to reuse the final answer from the first example. The indefinite integral is,

4 5 21 33 9 95 2

x x dx x x x c+ − = + − +∫

A couple of warnings are now in order. One of the more common mistakes that students make with integrals (both indefinite and definite) is to drop the dx at the end of the integral. This is required! Think of the integral sign and the dx as a set of parenthesis. You already know and are probably quite comfortable with the idea that every time you open a parenthesis you must close it. With integrals, think of the integral sign as an “open parenthesis” and the dx as a “close parenthesis”. If you drop the dx it won’t be clear where the integrand ends. Consider the following variations of the above example.

4 5 2

4 5 2

4 5

1 33 9 95 21 33 9 95 213 9 3 95

x x dx x x x c

x x dx x x c

x dx x x c x

+ − = + − +

+ − = + + −

+ − = + + −

∫

∫

∫

You only integrate what is between the integral sign and the dx. Each of the above integrals end in a different place and so we get different answers because we integrate a different number of terms each time. In the second integral the “-9” is outside the integral and so is left alone and not integrated. Likewise, in the third integral the “ 3 9x − ” is outside the integral and so is left alone.


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Knowing which terms to integrate is not the only reason for writing the dx down. In the Substitution Rule section we will actually be working with the dx in the problem and if we aren’t in the habit of writing it down it will be easy to forget about it and then we will get the wrong answer at that stage. The moral of this is to make sure and put in the dx! At this stage it may seem like a silly thing to do, but it just needs to be there, if for no other reason than knowing where the integral stops. On a side note, the dx notation should seem a little familiar to you. We saw things like this a couple of sections ago. We called the dx a differential in that section and yes that is exactly what it is. The dx that ends the integral is nothing more than a differential. The next topic that we should discuss here is the integration variable used in the integral. Actually there isn’t really a lot to discuss here other than to note that the integration variable doesn’t really matter. For instance,

4 5 2

4 5 2

4 5 2

1 33 9 95 2

1 33 9 95 2

1 33 9 95 2

x x dx x x x c

t t dt t t t c

w w dw w w w c

+ − = + − +

+ − = + − +

+ − = + − +

∫

∫

∫

Changing the integration variable in the integral simply changes the variable in the answer. It is important to notice however that when we change the integration variable in the integral we also changed the differential (dx, dt, or dw) to match the new variable. This is more important that we might realize at this point. Another use of the differential at the end of integral is to tell us what variable we are integrating with respect to. At this stage that may seem unimportant since most of the integrals that we’re going to be working with here will only involve a single variable. However, if you are on a degree track that will take you into multi-variable calculus this will be very important at that stage since there will be more than one variable in the problem. You need to get into the habit of writing the correct differential at the end of the integral so when it becomes important in those classes you will already be in the habit of writing it down. To see why this is important take a look at the following two integrals.

2 2x dx t dx∫ ∫ The first integral is simple enough.

22x dx x c= +∫


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The second integral is also fairly simple, but we need to be careful. The dx tells us that we are integrating x’s. That means that we only integrate x’s that are in the integrand and all other variables in the integrand are considered to be constants. The second integral is then,

2 2t dx tx c= +∫ So, it may seem silly to always put in the dx, but it is a vital bit of notation that can cause us to get the incorrect answer if we neglect to put it in. Now, there are some important properties of integrals that we should take a look at. Properties of the Indefinite Integral

1. ( ) ( )k f x dx k f x dx=∫ ∫ where k is any number. So, we can factor multiplicative constants out of indefinite integrals. See the Proof of Various Integral Formulas section of the Extras chapter to see the proof of this property.

2. ( ) ( )f x dx f x dx− = −∫ ∫ . This is really the first property with 1k = − and so no proof of this property will be given.

3. ( ) ( ) ( ) ( )f x g x dx f x dx g x dx± = ±∫ ∫ ∫ . In other words, the integral of a sum or difference of functions is the sum or difference of the individual integrals. This rule can be extended to as many functions as we need. See the Proof of Various Integral Formulas section of the Extras chapter to see the proof of this property.

Notice that when we worked the first example above we used the first and third property in the discussion. We integrated each term individually, put any constants back in and then put everything back together with the appropriate sign. Not listed in the properties above were integrals of products and quotients. The reason for this is simple. Just like with derivatives each of the following will NOT work.

( ) ( ) ( ) ( ) ( )( )

( )( )

f x dxf xf x g x dx f x dx g x dx dx

g x g x dx≠ ≠

⌠⌡

∫∫ ∫ ∫ ∫

With derivatives we had a product rule and a quotient rule to deal with these cases. However, with integrals there are no such rules. When faced with a product and quotient in an integral we will have a variety of ways of dealing with it depending on just what the integrand is.


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There is one final topic to be discussed briefly in this section. On occasion we will be given

( )f x′ and will ask what ( )f x was. We can now answer this question easily with an indefinite

integral.

( ) ( )f x f x dx′= ∫ Example 3 If ( ) 4 3 9f x x x′ = + − what was ( )f x ? Solution By this point in this section this is a simple question to answer.

( ) ( ) 4 5 21 33 9 95 2

f x f x dx x x dx x x x c′= = + − = + − +∫ ∫

In this section we kept evaluating the same indefinite integral in all of our examples. The point of this section was not to do indefinite integrals, but instead to get us familiar with the notation and some of the basic ideas and properties of indefinite integrals. The next couple of sections are devoted to actually evaluating indefinite integrals.


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Math 140


Computing Indefinite Integrals In the previous section we started looking at indefinite integrals and in that section we concentrated almost exclusively on notation, concepts and properties of the indefinite integral. In this section we need to start thinking about how we actually compute indefinite integrals. We’ll start off with some of the basic indefinite integrals. The first integral that we’ll look at is the integral of a power of x.

1

, 11

nn xx dx c n

n

+

= + ≠ −+∫

The general rule when integrating a power of x we add one onto the exponent and then divide by the new exponent. It is clear (hopefully) that we will need to avoid 1n = − in this formula. If we allow 1n = − in this formula we will end up with division by zero. We will take care of this case in a bit. Next is one of the easier integrals but always seems to cause problems for people.

, and are constantsk dx kx c c k= +∫ If you remember that all we’re asking is what did we differentiate to get the integrand this is pretty simple, but it does seem to cause problems on occasion. Let’s now take a look at the trig functions.

2

2

sin cos cos sin

sec tan sec tan sec

csc cot csc cot csc

x dx x c x dx x c

x dx x c x x dx x c

x dx x c x x dx x c

= − + = +

= + = +

= − + = − +

∫ ∫∫ ∫∫ ∫

Notice that we only integrated two of the six trig functions here. The remaining four integrals are really integrals that give the remaining four trig functions. Also, be careful with signs here. It is easy to get the signs for derivatives and integrals mixed up. Again, remember that we’re asking what function we differentiated to get the integrand. We will be able to integrate the remaining four trig functions in a couple of sections, but they all require the Substitution Rule. Now, let’s take care of exponential and logarithm functions.

11 lnln

xx x x adx c a dx c dx x dx x c

a x−= + = + = = +⌠

⌡∫ ∫ ∫e e


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Integrating logarithms requires a topic that is usually taught in Calculus II and so we won’t be integrating a logarithm in this class. Also note the third integrand can be written in a couple of ways and don’t forget the absolute value bars in the x in the answer to the third integral. Finally, let’s take care of the inverse trig and hyperbolic functions.

1 12 2

2

2

1 1tan sin1 1

sinh cosh cosh sinh

sech tanh sech tanh sech

csch coth csch coth csch

dx x c dx x cx x

x dx x c x dx x c

x dx x c x x dx x c

x dx x c x x dx x c

− −= + = ++ −

= + = +

= + = − +

= − + = − +

⌠⌠ ⌡ ⌡

∫ ∫∫ ∫∫ ∫

As with logarithms integrating inverse trig functions requires a topic usually taught in Calculus II and so we won’t be integrating them in this class. There is also a different answer for the second integral above. Recalling that since all we are asking here is what function did we differentiate to get the integrand the second integral could also be,

1

2

1 cos1

dx x cx

−= − +−

⌠⌡

Traditionally we use the first form of this integral. Okay, now that we’ve got most of the basic integrals out of the way let’s do some indefinite integrals. In all of these problems remember that we can always check our answer by differentiating and making sure that we get the integrand. Example 1 Evaluate each of the following indefinite integrals.

(a) 3 65 10 4t t dt−− +∫ [Solution]

(b) 8 8x x dx−+∫ [Solution]

(c) 345

7 136

x dxx x

+ +⌠⌡

[Solution]

(d) dy∫ [Solution]

(e) ( )( )23 4w w w dw+ −∫ [Solution]

(f) 10 4 2

3

4 2 15x x x dxx

− +⌠⌡

[Solution]

Solution Okay, in all of these remember the basic rules of indefinite integrals. First, to integrate sums and differences all we really do is integrate the individual terms and then put the terms back together


Math 140


with the appropriate signs. Next, we can ignore any coefficients until we are done with integrating that particular term and then put the coefficient back in. Also, do not forget the “+c” at the end it is important and must be there. So, let’s evaluate some integrals.

(a) 3 65 10 4t t dt−− +∫

There’s not really a whole lot to do here other than use the first two formulas from the beginning of this section. Remember that when integrating powers (that aren’t -1 of course) we just add one onto the exponent and then divide by the new exponent.

3 6 4 5

4 5

1 15 10 4 5 10 44 5

5 2 44

t t dt t t t c

t t t c

− −

−

− + = − + + −

= + + +

∫

Be careful when integrating negative exponents. Remember to add one onto the exponent. One of the more common mistakes that students make when integrating negative exponents is to “add one” and end up with an exponent of “-7” instead of the correct exponent of “-5”.

[Return to Problems]

(b) 8 8x x dx−+∫

This is here just to make sure we get the point about integrating negative exponents.

8 8 9 71 19 7

x x dx x x c− −+ = − +∫


(c) 345

7 136

x dxx x

+ +⌠⌡

In this case there isn’t a formula for explicitly dealing with radicals or rational expressions. However, just like with derivatives we can write all these terms so they are in the numerator and they all have an exponent. This should always be your first step when faced with this kind of integral just as it was when differentiating.

3 13 54 4 2

5

7 144 2

7 144 2

7 1 13 3 766

1 7 1 13 7 14 6 2412 7 17 4 3

x dx x x x dxx x

x x x c

x x x c

−−

−

−

+ + = + +

= − + +

= − + +

⌠ ⌠ ⌡ ⌡

When dealing with fractional exponents we usually don’t “divide by the new exponent”. Doing


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this is equivalent to multiplying by the reciprocal of the new exponent and so that is what we will usually do.


(d) dy∫

Don’t make this one harder than it is…

1dy dy y c= = +∫ ∫ In this case we are really just integrating a one!


(e) ( )( )23 4w w w dw+ −∫

We’ve got a product here and as we noted in the previous section there is no rule for dealing with products. However, in this case we don’t need a rule. All that we need to do is multiply things out (taking care of the radicals at the same time of course) and then we will be able to integrate.

( )( )

1 72 33 3 3

4 102 4 3 3

4 4 4

1 32 34 10

w w w dw w w w w dw

w w w w c

+ − = − + −

= − + − +

∫ ∫


(f) 10 4 2

3

4 2 15x x x dxx

− +⌠⌡

As with the previous part it’s not really a problem that we don’t have a rule for quotients for this integral. In this case all we need to do is break up the quotient and then integrate the individual terms.

10 4 2 10 4 2

3 3 3 3

7

8 2

4 2 15 4 2 15

154 2

1 15ln2

x x x x x xdx dxx x x x

x x dxx

x x x c

− += − +

= − +

= − + +

⌠ ⌠ ⌡ ⌡

⌠⌡

Be careful to not think of the third term as x to a power for the purposes of integration. Using that rule on the third term will NOT work. The third term is simply a logarithm. Also, don’t get excited about the 15. The 15 is just a constant and so it can be factored out of the integral. In other words, here is what we did to integrate the third term.

15 115 15lndx dx x cx x

= = +⌠ ⌠ ⌡ ⌡



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Always remember that you can’t integrate products and quotients in the same way that we integrate sums and differences. At this point the only way to integrate products and quotients is to multiply the product out or break up the quotient. Eventually we’ll see some other products and quotients that can be dealt with in other ways. However, there will never be a single rule that will work for all products and there will never be a single rule that will work for all quotients. Every product and quotient is different and will need to be worked on a case by case basis. The first set of examples focused almost exclusively on powers of x (or whatever variable we used in each example). It’s time to do some examples that involve other functions. Example 2 Evaluate each of the following integrals.

(a) 23 5cos 10secx x x dx+ −∫ e [Solution]

(b) 12sec tan6

w w dww

+⌠⌡

[Solution]

(c) 2

23 96csc cot1

y y dyy y

+ ++

⌠⌡

[Solution]

(d) 2

3 6sin 10sinh1

x x dxx

+ +−

⌠⌡

[Solution]

(e) 2

2

7 6sinsin

dθθ

θ−⌠

⌡

[Solution]

Solution Most of the problems in this example will simply use the formulas from the beginning of this section. More complicated problems involving most of these functions will need to wait until we reach the Substitution Rule.

(a) 23 5cos 10secx x x dx+ −∫ e

There isn’t anything to this one other than using the formulas.

23 5cos 10sec 3 5sin 10 tanx xx x dx x x c+ − = + − +∫ e e [Return to Problems]

(b) 12sec tan

6w w dw

w+⌠

⌡

Let’s be a little careful with this one. First break it up into two integrals and note the rewritten integrand on the second integral.

1 1 12sec tan 2sec tan6 6

1 12sec tan6

w w dw w wdw dww w

w wdw dww

+ = +

= +

⌠ ⌠ ⌡ ⌡

⌠⌡

∫

∫


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Rewriting the second integrand will help a little with the integration at this early stage. We can think of the 6 in the denominator as a 1/6 out in front of the term and then since this is a constant it can be factored out of the integral. The answer is then,

1 12sec tan 2sec ln6 6

w w dw w w cw

+ = + +⌠⌡

Note that we didn’t factor the 2 out of the first integral as we factored the 1/6 out of the second. In fact, we will generally not factor the 1/6 out either in later problems. It was only done here to make sure that you could follow what we were doing.


(c) 2

23 96csc cot1

y y dyy y

+ ++

⌠⌡

In this one we’ll just use the formulas from above and don’t get excited about the coefficients. They are just multiplicative constants and so can be ignored while we integrate each term and then once we’re done integrating a given term we simply put the coefficients back in.

12

23 96csc cot 23tan 6csc 9ln1

y y dy y y y cy y

−+ + = − + ++

⌠⌡


(d) 2

3 6sin 10sinh1

x x dxx

+ +−

⌠⌡

Again, there really isn’t a whole lot to do with this one other than to use the appropriate formula from above.

1

2

3 6sin 10sinh 3sin 6cos 10cosh1

x x dx x x x cx

−+ + = − + +−

⌠⌡


(e) 2

2

7 6sinsin

dθθ

θ−⌠

⌡

This one can be a little tricky if you aren’t ready for it. As discussed previously, at this point the only way we have of dealing with quotients is to break it up.

2

2 2

2

7 6sin 7 6sin sin

7csc 6

d d

d

θθ θ

θ θ

θ θ

−= −

= −

⌠ ⌠ ⌡⌡

∫

Notice that upon breaking the integral up we further simplified the integrand by recalling the definition of cosecant. With this simplification we can do the integral.

2

2

7 6sin 7 cot 6sin

d cθθ θ θ

θ−

= − − +⌠⌡



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As shown in the last part of this example we can do some fairly complicated looking quotients at this point if we remember to do simplifications when we see them. In fact, this is something that you should always keep in mind. In almost any problem that we’re doing here don’t forget to simplify where possible. In almost every case this can only help the problem and will rarely complicate the problem. In the next problem we’re going to take a look at a product and this time we’re not going to be able to just multiply the product out. However, if we recall the comment about simplifying a little this problem becomes fairly simple.

Example 3 Integrate sin cos2 2t t dt

⌠⌡

.

Solution There are several ways to do this integral and most of them require the next section. However, there is a way to do this integral using only the material from this section. All that is required is to remember the trig formula that we can use to simplify the integrand up a little. Recall the following double angle formula. ( )sin 2 2sin cost t t= A small rewrite of this formula gives,

( )12sin cos sin 2t t t=

If we now replace all the t’s with 2

t we get,

( )1sin cos sin2 2 2t t t =

Using this formula the integral becomes something we can do.

( )

( )

1sin cos sin2 2 2

1 cos2

t t dt t dt

t c

=

= − +

⌠ ⌠ ⌡⌡

As noted earlier there is another method for doing this integral. In fact there are two alternate methods. To see all three check out the section on Constant of Integration in the Extras chapter but be aware that the other two do require the material covered in the next section. The formula/simplification in the previous problem is a nice “trick” to remember. It can be used on occasion to greatly simplify some problems. There is one more set of examples that we should do before moving out of this section.


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Example 4 Given the following information determine the function ( )f x .

(a) ( ) ( )34 9 2sin 7 , 0 15xf x x x f′ = − + + =e [Solution]

(b) ( ) ( ) ( )3 515 5 6, 1 , 4 4044

f x x x f f′′ = + + = − = [Solution]

Solution In both of these we will need to remember that

( ) ( )f x f x dx′= ∫ Also note that because we are giving values of the function at specific points we are also going to be determining what the constant of integration will be in these problems. (a) ( ) ( )34 9 2sin 7 , 0 15xf x x x f′ = − + + =e

The first step here is to integrate to determine the most general possible ( )f x .

( ) 3

4

4 9 2sin 7

9 2cos 7

x

x

f x x x dx

x x x c

= − + +

= − − + +∫ e

e

Now we have a value of the function so let’s plug in 0x = and determine the value of the constant of integration c.

( ) ( ) ( )4 015 0 0 9 0 2cos 0 7

2 75

f cc

c

= = − − + +

= − + += +

e

So, from this it looks like 10c = . This means that the function is, ( ) 4 9 2cos 7 10xf x x x x= − − + +e


(b) ( ) ( ) ( )3 515 5 6, 1 , 4 4044

f x x x f f′′ = + + = − =

This one is a little different form the first one. In order to get the function we will need the first derivative and we have the second derivative. We can however, use an integral to get the first derivative from the second derivative, just as we used an integral to get the function from the first derivative. So, let’s first get the most general possible first derivative.


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( ) ( )1

32

342

342

15 5 6

2 515 63 4

510 64

f x f x dx

x x dx

x x x c

x x x c

′ ′′=

= + +

= + + +

= + + +

∫

∫

Don’t forget the constant of integration! We can now find the most general possible function.

( )

342

55 22

510 64

14 34

f x x x x c dx

x x x cx d

= + + +

= + + + +

⌠⌡

Do not get excited about integrating the c. It’s just a constant and we know how to integrate constants. Also, there will be no reason to think the constants of integration from the integration in each step will be the same and so we’ll need to call each constant of integration something different. Now, plug in the two values of the function that we’ve got.

( )

( ) ( ) ( ) ( ) ( )

5 1 291 4 34 4 4

1404 4 4 32 1024 3 16 4 432 44

f c d c d

f c d c d

− = = + + + + = + +

= = + + + + = + +

This gives us a system of two equations in two unknowns that we can solve.

5 29 134 4 2

404 432 4 2

c d c

c d d

− = + + = −⇒

= + + = −

The function is then,

( )5

5 22 1 134 3 24 2

f x x x x x= + + − −

Don’t remember how to solve systems? Check out the Solving Systems portion of my Algebra/Trig Review.

[Return to Problems] In this section we’ve started the process of integration. We’ve seen how to do quite a few basic integrals and we also saw a quick application of integrals in the last example.


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There are many new formulas in this section that we’ll now have to know. However, if you think about it, they aren’t really new formulas. They are really nothing more than derivative formulas that we should already know written in terms of integrals. If you remember that you should find it easier to remember the formulas in this section. Always remember that integration is asking nothing more than what function did we differentiate to get the integrand. If you can remember that many of the basic integrals that we saw in this section and many of the integrals in the coming sections aren’t too bad.


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Substitution Rule for Indefinite Integrals After the last section we now know how to do the following integrals.

43

1 cos yx dx dt wdw dyt

⌠⌡∫ ∫ ∫e

However, we can’t do the following integrals.

( )( ) ( ) 2

32 34

34

4

2 118 6 52

11 cos ln 8 1 y y

tx x dx dtt t

w w dw y dyw

−

++

+

− − −

⌠⌡

⌠⌡

∫

∫ e

All of these look considerably more difficult than the first set. However, they aren’t too bad once you see how to do them. Let’s start with the first one.

2 3418 6 5x x dx+∫ In this case let’s notice that if we let 36 5u x= + and we compute the differential (you remember how to compute these right?) for this we get, 218du x dx= Now, let’s go back to our integral and notice that we can eliminate every x that exists in the integral and write the integral completely in terms of u using both the definition of u and its differential.

( ) ( )

12 3 3 24 4

14

18 6 5 6 5 18x x dx x x dx

u du

+ = +

=

⌠⌡∫

∫

In the process of doing this we’ve taken an integral that looked very difficult and with a quick substitution we were able to rewrite the integral into a very simple integral that we can do. Evaluating the integral gives,

( )1 5 5

2 3 34 4 4 44 418 6 5 6 55 5

x x dx u du u c x c+ = = + = + +∫ ∫

As always we can check our answer with a quick derivative if we’d like to and don’t forget to “back substitute” and get the integral back into terms of the original variable. What we’ve done in the work above is called the Substitution Rule. Here is the substitution rule in general.


Math 140


Substitution Rule

( )( ) ( ) ( ) ( ), where, f g x g x dx f u du u g x′ = =∫ ∫ A natural question at this stage is how to identify the correct substitution. Unfortunately, the answer is it depends on the integral. However, there is a general rule of thumb that will work for many of the integrals that we’re going to be running across. When faced with an integral we’ll ask ourselves what we know how to integrate. With the integral above we can quickly recognize that we know how to integrate

4 x dx∫

However, we didn’t have just the root we also had stuff in front of the root and (more importantly in this case) stuff under the root. Since we can only integrate roots if there is just an x under the root a good first guess for the substitution is then to make u be the stuff under the root. Another way to think of this is to ask yourself what portion of the integrand has an inside function and can you do the integral with that inside function present. If you can’t then there is a pretty good chance that the inside function will be the substitution. We will have to be careful however. There are times when using this general rule can get us in trouble or overly complicate the problem. We’ll eventually see problems where there are more than one “inside function” and/or integrals that will look very similar and yet use completely different substitutions. The reality is that the only way to really learn how to do substitutions is to just work lots of problems and eventually you’ll start to get a feel for how these work and you’ll find it easier and easier to identify the proper substitutions. Now, with that out of the way we should ask the following question. How, do we know if we got the correct substitution? Well, upon computing the differential and actually performing the substitution every x in the integral (including the x in the dx) must disappear in the substitution process and the only letters left should be u’s (including a du). If there are x’s left over then there is a pretty good chance that we chose the wrong substitution. Unfortunately, however there is at least one case (we’ll be seeing an example of this in the next section) where the correct substitution will actually leave some x’s and we’ll need to do a little more work to get it to work. Again, it cannot be stressed enough at this point that the only way to really learn how to do substitutions is to just work lots of problems. There are lots of different kinds of problems and after working many problems you’ll start to get a real feel for these problems and after you work enough of them you’ll reach the point where you’ll be able to do simple substitutions in your head without having to actually write anything down. As a final note we should point out that often (in fact in almost every case) the differential will not appear exactly in the integrand as it did in the example above and sometimes we’ll need to do


Math 140


some manipulation of the integrand and/or the differential to get all the x’s to disappear in the substitution. Let’s work some examples so we can get a better idea on how the substitution rule works. Example 1 Evaluate each of the following integrals.

(a) ( )11 cos lnw w dww

− −

⌠⌡

[Solution]

(b) ( ) 243 8 1 y yy dy−−∫ e [Solution]

(c) ( )42 33 10x x dx−∫ [Solution]

(d) 21 4

x dxx−

⌠⌡

[Solution]

Solution

(a) ( )11 cos lnw w dww

− −

⌠⌡

In this case we know how to integrate just a cosine so let’s make the substitution the stuff that is inside the cosine.

1ln 1u w w du dww

= − = −

So, as with the first example we worked the stuff in front of the cosine appears exactly in the differential. The integral is then,

( ) ( )

( )( )

11 cos ln cos

sin

sin ln

w w dw u duw

u c

w w c

− − =

= +

= − +

⌠⌡ ∫

Don’t forget to go back to the original variable in the problem.


(b) ( ) 243 8 1 y yy dy−−∫ e

Again, we know how to integrate an exponential by itself so it looks like the substitution for this problem should be,

( )24 8 1u y y du y dy= − = −

Now, with the exception of the 3 the stuff in front of the exponential appears exactly in the differential. Recall however that we can factor the 3 out of the integral and so it won’t cause any problems. The integral is then,


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( ) 2

2

4

4

3 8 1 3

3

3

y y u

u

y y

y dy du

c

c

−

−

− =

= +

= +

∫ ∫e e

e

e


(c) ( )42 33 10x x dx−∫

In this case it looks like the following should be the substitution. 3 23 10 30u x du x dx= − = − Okay, now we have a small problem. We’ve got an x2 out in front of the parenthesis but we don’t have a “-30”. This is not really the problem it might appear to be at first. We will simply rewrite the differential as follows.

2 130

x dx du= −

With this we can now substitute the “x2 dx” away. In the process we will pick up a constant, but that isn’t a problem since it can always be factored out of the integral. We can now do the integral.

( ) ( )

( )

4 42 3 3 2

4

5

53

3 10 3 10

130

1 130 51 3 10

150

x x dx x x dx

u du

u c

x c

− = −

= − = − +

= − − +

⌠⌡

∫ ∫

Note that in most problems when we pick up a constant as we did in this example we will generally factor it out of the integral in the same step that we substitute it in.


(d) 21 4

x dxx−

⌠⌡

In this example don’t forget to bring the root up to the numerator and change it into fractional exponent form. Upon doing this we can see that the substitution is,

2 11 4 88

u x du x dx xdx du= − = − ⇒ = −

The integral is then,


Math 140


( )

( )

12 2

2

12

12

12 2

1 41 4

18141 1 44

x dx x x dxx

u du

u c

x c

−

−

= −−

= −

= − +

= − − +

⌠⌡ ∫

∫

[Return to Problems] In the previous set of examples the substitution was generally pretty clear. There was exactly one term that had an “inside function” that we also couldn’t integrate. Let’s take a look at some more complicated problems to make sure we don’t come to expect all substitutions are like those in the previous set of examples. Example 2 Evaluate each of the following integrals.

(a) ( ) ( )( )4sin 1 2 cos 1x x dx− − −∫ [Solution]

(b) ( ) ( )10cos 3 sin 3z z dz∫ [Solution]

(c) ( ) ( )( )32sec 4 3 tan 4t t dt−∫ [Solution] Solution

(a) ( ) ( )( )4sin 1 2 cos 1x x dx− − −∫

In this problem there are two “inside functions”. There is the 1 x− that is inside the two trig functions and there is also the term that is raised to the 4th power. There are two ways to proceed with this problem. The first idea that many students have is substitute the 1 x− away. There is nothing wrong with doing this but it doesn’t eliminate the problem of the term to the 4th power. That’s still there and if we used this idea we would then need to do a second substitution to deal with that. The second (and much easier) way of doing this problem is to just deal with the stuff raised to the 4th power and see what we get. The substitution in this case would be, ( ) ( ) ( )2 cos 1 sin 1 sin 1u x du x dx x dx du= − − = − − ⇒ − = −

Two things to note here. First, don’t forget to correctly deal with the “-”. A common mistake at this point is to lose it. Secondly, notice that the 1 x− turns out to not really be a problem after all. Because the 1 x− was “buried” in the substitution that we actually used it was also taken care of at the same time. The integral is then,


Math 140


( ) ( )( )

( )( )

4 4

5

5

sin 1 2 cos 1

151 2 cos 15

x x dx u du

u c

x c

− − − = −

= − +

= − − − +

∫ ∫

As seen in this example sometimes there will seem to be two substitutions that will need to be done however, if one of them is buried inside of another substitution then we’ll only really need to do one. Recognizing this can save a lot of time in working some of these problems.


(b) ( ) ( )10cos 3 sin 3z z dz∫

This one is a little tricky at first. We can see the correct substitution by recalling that,

( ) ( )( )1010sin 3 sin 3z z= Using this it looks like the correct substitution is,

( ) ( ) ( ) 1sin 3 3cos 3 cos 33

u z du z dz z dz du= = ⇒ =

Notice that we again had two apparent substitutions in this integral but again the 3z is buried in the substitution we’re using and so we didn’t need to worry about it. Here is the integral.

( ) ( )

( )

10 10

11

11

1cos 3 sin 331 13 111 sin 333

z z dz u du

u c

z c

=

= +

= +

∫ ∫

Note that the one third in front of the integral came about from the substitution on the differential and we just factored it out to the front of the integral. This is what we will usually do with these constants.


(c) ( ) ( )( )32sec 4 3 tan 4t t dt−∫

In this case we’ve got a 4t, a secant squared as well as a term cubed. However, it looks like if we use the following substitution the first two issues are going to be taken care of for us.

( ) ( ) ( )2 2 13 tan 4 4sec 4 sec 44

u t du t dt t dt du= − = − ⇒ = −


Math 140


The integral is now,

( ) ( )( )

( )( )

32 3

4

4

1sec 4 3 tan 441

161 3 tan 4

16

t t dt u du

u c

t c

− = −

= − +

= − − +

∫ ∫

[Return to Problems] The most important thing to remember in substitution problems is that after the substitution all the original variables need to disappear from the integral. After the substitution the only variables that should be present in the integral should be the new variable from the substitution (usually u). Note as well that this includes the variables in the differential! This next set of examples, while not particular difficult, can cause trouble if we aren’t paying attention to what we’re doing. Example 3 Evaluate each of the following integrals.

(a) 35 4

dyy +

⌠⌡

[Solution]

(b) 2

35 4

y dyy +

⌠⌡

[Solution]

(c) ( )22

3

5 4

y dyy +

⌠⌡

[Solution]

(d) 2

35 4

dyy +

⌠⌡

[Solution]

Solution

(a) 3

5 4dy

y +⌠⌡

We haven’t seen a problem quite like this one yet. Let’s notice that if we take the denominator and differentiate it we get just a constant and the only thing that we have in the numerator is also a constant. This is a pretty good indication that we can use the denominator for our substitution so,

15 4 55

u y du dy dy du= + = ⇒ =



Math 140


3 3 15 4 5

3 ln53 ln 5 45

dy duy u

u c

y c

=+

= +

= + +

⌠ ⌠ ⌡⌡

Remember that we can just factor the 3 in the numerator out of the integral and that makes the integral a little clearer in this case.


(b) 2

35 4

y dyy +

⌠⌡

The integral is very similar to the previous one with a couple of minor differences but notice that again if we differentiate the denominator we get something that is different from the numerator by only a multiplicative constant. Therefore we’ll again take the denominator as our substitution.

2 15 4 1010

u y du y dy y dy du= + = ⇒ =

The integral is,

2

2

3 3 15 4 10

3 ln103 ln 5 4

10

y dy duy u

u c

y c

=+

= +

= + +

⌠ ⌠ ⌡⌡


(c) ( )22

3

5 4

y dyy +

⌠⌡

Now, this one is almost identical to the previous part except we added a power onto the denominator. Notice however that if we ignore the power and differentiate what’s left we get the same thing as the previous example so we’ll use the same substitution here.

2 15 4 1010

u y du y dy y dy du= + = ⇒ =

The integral in this case is,


Math 140


( )

( ) ( )

222

1

122

3 3105 4

3103 35 4

10 10 5 4

y dy u duy

u c

y c cy

−

−

−

=+

= − +

= − + + = − ++

⌠⌡

∫

Be careful in this case to not turn this into a logarithm. After working problems like the first two in this set a common error is to turn every rational expression into a logarithm. If there is a power on the whole denominator then there is a good chance that it isn’t a logarithm. The idea that we used in the last three parts to determine the substitution is not a bad idea to remember. If we’ve got a rational expression try differentiating the denominator (ignoring any powers that are on the whole denominator) and if the result is the numerator or only differs from the numerator by a multiplicative constant then we can usually use that as our substitution.


(d) 2

35 4

dyy +

⌠⌡

Now, this part is completely different from the first three and yet seems similar to them as well. In this case if we differentiate the denominator we get a y that is not in the numerator and so we can’t use the denominator as our substitution. In fact, because we have y2 in the denominator and no y in the numerator is an indication of how to work this problem. This integral is going to be an inverse tangent when we are done. To key to seeing this is to recall the following formula,

12

1 tan1

du u cu

−= ++

⌠⌡

We clearly don’t have exactly this but we do have something that is similar. The denominator has a squared term plus a constant and the numerator is just a constant. So, with a little work and the proper substitution we should be able to get our integral into a form that will allow us to use this formula. There is one part of this formula that is really important and that is the “1+” in the denominator. That must be there and we’ve got a “4+” but it is easy enough to take care of that. We’ll just factor a 4 out of the denominator and at the same time we’ll factor the 3 in the numerator out of the integral as well. Doing this gives,


Math 140


2 2

2

2

3 35 4 54 1

4

3 154 1

4

3 14 5 1

2

dy dyy y

dyy

dyy

=+

+

=+

=

+

⌠⌠ ⌡

⌡

⌠⌡

⌠⌡

Notice that in the last step we rewrote things a little in the denominator. This will help us to see what the substitution needs to be. In order to get this integral into the formula above we need to end up with a u2 in the denominator. Our substitution will then need to be something that upon

squaring gives us 25

4y . With the rewrite we can see what that we’ll need to use the following

substitution.

5 5 22 2 5

yu du dy dy du= = ⇒ =

Don’t get excited about the root in the substitution, these will show up on occasion. Upon plugging our substitution in we get,

2 2

3 3 2 15 4 4 15

dy duy u

= + + ⌠ ⌠ ⌡⌡

After doing the substitution, and factoring any constants out, we get exactly the integral that gives an inverse tangent and so we know that we did the correct substitution for this integral. The integral is then,

( )

2 2

1

1

3 3 15 4 12 5

3 tan2 5

3 5tan22 5

dy duy u

u c

y c

−

−

=+ +

= +

= +

⌠ ⌠ ⌡⌡

[Return to Problems] In this last set of integrals we had four integrals that were similar to each other in many ways and yet all either yielded different answer using the same substitution or used a completely different substitution than one that was similar to it.


Math 140


This is a fairly common occurrence and so you will need to be able to deal with these kinds of issues. There are many integrals that on the surface look very similar and yet will use a completely different substitution or will yield a completely different answer when using the same substitution. Let’s take a look at another set of examples to give us more practice in recognizing these kinds of issues. Note however that we won’t be putting as much detail into these as we did with the previous examples. Example 4 Evaluate each of the following integrals.

(a) ( )

3

34

2 1

2

t dtt t

+

+

⌠⌡

[Solution]

(b) 3

4

2 12

t dtt t

++

⌠⌡

[Solution]

(c) 21 4

x dxx−

⌠⌡

[Solution]

(d) 2

11 4

dxx−

⌠⌡

[Solution]

Solution

(a) ( )

3

34

2 1

2

t dtt t

+

+

⌠⌡

Clearly the derivative of the denominator, ignoring the exponent, differs from the numerator only by a multiplicative constant and so the substitution is,

( ) ( ) ( )4 3 3 3 12 4 2 2 2 1 2 12

u t t du t dt t dt t dt du= + = + = + ⇒ + =

After a little manipulation of the differential we get the following integral.

( )

( )

3

3 34

3

2

24

2 1 1 122

121 12 2

1 24

t dt duut t

u du

u c

t t c

−

−

−

+=

+

=

= − +

= − + +

⌠ ⌠ ⌡⌡

∫



Math 140


(b) 3

4

2 12

t dtt t

++

⌠⌡

The only difference between this problem and the previous one is the denominator. In the previous problem the whole denominator is cubed and in this problem the denominator has no power on it. The same substitution will work in this problem but because we no longer have the power the problem will be different. So, using the substitution from the previous example the integral is,

3

4

4

2 1 1 12 2

1 ln21 ln 22

t dt dut t u

u c

t t c

+=

+

= +

= + +

⌠ ⌠ ⌡⌡

So, in this case we get a logarithm from the integral. [Return to Problems]

(c) 21 4

x dxx−

⌠⌡

Here, if we ignore the root we can again see that the derivative of the stuff under the radical differs from the numerator by only a multiplicative constant and so we’ll use that as the substitution.

2 11 4 88

u x du x dx x dx du= − = − ⇒ = −


( )

12

2

12

2

181 41 281 1 44

x dx u dux

u c

x c

−= −

−

= − +

= − − +

⌠⌡ ∫


(d) 2

11 4

dxx−

⌠⌡

In this case we are missing the x in the numerator and so the substitution from the last part will do us no good here. This integral is another inverse trig function integral that is similar to the last part of the previous set of problems. In this case we need to following formula.


Math 140


1

2

1 sin1

du u cu

−= +−

⌠⌡

The integral in this problem is nearly this. The only difference is the presence of the coefficient of 4 on the x2. With the correct substitution this can be dealt with however. To see what this substitution should be let’s rewrite the integral a little. We need to figure out what we squared to get 4x2 and that will be our substitution.

( )2 2

1 11 4 1 2

dx dxx x

=− −

⌠⌠

⌡ ⌡

With this rewrite it looks like we can use the following substitution.

12 22

u x du dx dx du= = ⇒ =


( )

2 2

1

1

1 1 121 4 11 sin21 sin 22

dx dux u

u c

x c

−

−

=− −

= +

= +

⌠ ⌠ ⌡ ⌡

[Return to Problems] Since this document is also being presented on the web we’re going to put the rest of the substitution rule examples in the next section. With all the examples in one section the section was becoming too large for web presentation.


Math 140


More Substitution Rule In order to allow these pages to be displayed on the web we’ve broken the substitution rule examples into two sections. The previous section contains the introduction to the substitution rule and some fairly basic examples. The examples in this section tend towards the slightly more difficult side. Also, we’ll not be putting quite as much explanation into the solutions here as we did in the previous section. In the first couple of sets of problems in this section the difficulty is not with the actual integration itself, but with the set up for the integration. Most of the integrals are fairly simple and most of the substitutions are fairly simple. The problems arise in getting the integral set up properly for the substitution(s) to be done. Once you see how these are done it’s easy to see what you have to do, but the first time through these can cause problems if you aren’t on the lookout for potential problems. Example 1 Evaluate each of the following integrals.

(a) ( ) ( )2 sec 2 tan 2t t t dt+∫e [Solution]

(b) ( ) ( ) ( )( )3 2sin 4cos 6cos 8t t t dt+ −∫ [Solution]

(c) ( )22cos 1

1xx x dx

x+ +

+⌠⌡

[Solution]

Solution (a) ( ) ( )2 sec 2 tan 2t t t dt+∫e

This first integral has two terms in it and both will require the same substitution. This means that we won’t have to do anything special to the integral. One of the more common “mistakes” here is to break the integral up and do a separate substitution on each part. This isn’t really mistake but will definitely increase the amount of work we’ll need to do. So, since both terms in the integral use the same substitution we’ll just do everything as a single integral using the following substitution.

12 22

u t du dt dt du= = ⇒ =


( ) ( ) ( ) ( )

( )( )

( )( )

2

2

1sec 2 tan 2 sec tan21 sec21 sec 22

t u

u

t

t t dt u u du

u c

t c

+ = +

= + +

= + +

∫ ∫e e

e

e

Often a substitution can be used multiple times in an integral so don’t get excited about that if it happens. Also note that since there was a 1

2 in front of the whole integral there must also be a 12

in front of the answer from the integral. [Return to Problems]


Math 140


(b) ( ) ( ) ( )( )3 2sin 4cos 6cos 8t t t dt+ −∫

This integral is similar to the previous one, but it might not look like it at first glance. Here is the substitution for this problem, ( ) ( ) ( )cos sin sinu t du t dt t dt du= = − ⇒ = − We’ll plug the substitution into the problem twice (since there are two cosines) and will only work because there is a sine multiplying everything. Without that sine in front we would not be able to use this substitution. The integral in this case is,

( ) ( ) ( )( )( )

( ) ( ) ( )( )

3 2 3 2

4 3

4 3

sin 4cos 6cos 8 4 6 8

2 8

cos 2cos 8cos

t t t dt u u du

u u u c

t t t c

+ − = − + −

= − + − +

= − + − +

∫ ∫

Again, be careful with the minus sign in front of the whole integral.


(c) ( )22cos 1

1xx x dx

x+ +

+⌠⌡

It should be fairly clear that each term in this integral will use the same substitution, but let’s rewrite things a little to make things really clear.

( ) ( )2 22 2

1cos 1 cos 11 1

xx x dx x x dxx x

+ + = + + + + ⌠⌠ ⌡ ⌡

Since each term had an x in it and we’ll need that for the differential we factored that out of both terms to get it into the front. This integral is now very similar to the previous one. Here’s the substitution.

2 11 22

u x du x dx x dx du= + = ⇒ =

The integral is,

( ) ( )

( )( )

( )( )

22

2 2

1 1cos 1 cos1 2

1 sin ln21 sin 1 ln 12

xx x dx u dux u

u u c

x x c

+ + = ++

= + +

= + + + +

⌠ ⌠ ⌡ ⌡



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So, as we’ve seen in the previous set of examples sometimes we can use the same substitution more than once in an integral and doing so will simplify the work. Example 2 Evaluate each of the following integrals.

(a) 2 1 xx dx−+∫ e [Solution]

(b) ( )22

1cos 11

x x dxx

+ ++

⌠⌡

[Solution]

Solution (a) 2 1 xx dx−+∫ e

In this integral the first term does not need any substitution while the second term does need a substitution. So, to deal with that we’ll need to split the integral up as follows,

2 1 2 1x xx dx x dx dx− −+ = +∫ ∫ ∫e e The substitution for the second integral is then, 1u x du dx dx du= − = − ⇒ = − The integral is,

2 1 2

3

3 1

1313

x u

u

x

x dx x dx du

x c

x c

−

−

+ = −

= − +

= − +

∫ ∫ ∫e e

e

e

Be careful with this kind of integral. One of the more common mistakes here is do the following “shortcut”.

2 1 2x ux dx x du−+ = − +∫ ∫e e

In other words, some students will try do the substitution just the second term without breaking up the integral. There are two issues with this. First, there is a “-” in front of the whole integral that shouldn’t be there. It should only be on the second term because that is the term getting the substitution. Secondly, and probably more importantly, there are x’s in the integral and we have a du for the differential. We can’t mix variables like this. When we do integrals all the variables in the integrand must match the variable in the differential.


(b) ( )22

1cos 11

x x dxx

+ ++

⌠⌡

This integral looks very similar to Example 1c above, but it is different. In this integral we no longer have the x in the numerator of the second term and that means that the substitution we’ll use for the first term will no longer work for the second term. In fact,


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the second term doesn’t need a substitution at all since it is just an inverse tangent. The substitution for the first term is then,

2 11 22

u x du x dx x dx du= + = ⇒ =

Now let’s do the integral. Remember to first break it up into two terms before using the substitution.

( ) ( )

( )

( ) ( )

( ) ( )

2 22 2

2

1

2 1

1 1cos 1 cos 11 1

1 1cos2 11 sin tan21 sin 1 tan2

x x dx x x dx dxx x

u du dxx

u x c

x x c

−

−

+ + = + ++ +

= ++

= + +

= + + +

⌠ ⌠ ⌡ ⌡

⌠⌡

∫

∫

[Return to Problems] In this set of examples we saw that sometimes one (or potentially more than one) term in the integrand will not require a substitution. In these cases we’ll need to break up the integral into two integrals, one involving the terms that don’t need a substitution and another with the term(s) that do need a substitution. Example 3 Evaluate each of the following integrals.

(a) 2sec10

z z dz− +

⌠⌡

e [Solution]

(b) 1sin 1 2cos7 2

w w dww

− ++

⌠⌡

[Solution]

(c) 2

10 316

x dxx

++

⌠⌡

[Solution]

Solution

(a) 2sec10

z z dz− +

⌠⌡

e

In this integral, unlike any integrals that we’ve yet done, there are two terms and each will require a different substitution. So, to do this integral we’ll first need to split up the integral as follows,

2 2sec sec10 10

z zz zdz dz dz− − + = +

⌠ ⌠ ⌡ ⌡∫e e

Here are the substitutions for each integral.


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1 1010 10

u z du dz dz duzv dv dz dz dv

= − = − ⇒ = −

= = ⇒ =

Notice that we used different letters for each substitution to avoid confusion when we go to plug back in for u and v. Here is the integral.

( )

( )

2 2sec 10 sec10

10 tan

10 tan10

z u

u

z

z dz du v dv

v c

z c

−

−

+ = − +

= − + +

= − + +

⌠⌡ ∫ ∫e e

e

e


(b) 1sin 1 2cos

7 2w w dw

w− +

+⌠⌡

As with the last problem this integral will require two separate substitutions. Let’s first break up the integral.

( )12

1 1sin 1 2cos sin 1 2cos7 2 7 2

w w dw w w dw dww w

− + = − ++ +

⌠ ⌠ ⌡ ⌡∫

Here are the substitutions for this integral.

( ) ( ) ( ) 11 2cos 2sin sin

217 2 77

u w du w dw w dw du

v w dv dw dw dv

= − = ⇒ =

= + = ⇒ =


( )

12

32

32

1 1 1 1sin 1 2cos7 2 2 7

1 2 1 ln2 3 71 11 2cos ln 7 23 7

w w dw u du dvw v

u v c

w w c

− + = ++

= + +

= − + + +

⌠ ⌠ ⌡ ⌡∫


(c) 2

10 316

x dxx

++

⌠⌡

The last problem in this set can be tricky. If there was just an x in the numerator we could do a quick substitution to get a natural logarithm. Likewise if there wasn’t an x in the numerator we


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would get an inverse tangent after a quick substitution. To get this integral into a form that we can work with we will first need to break it up as follows.

2 2 2

22

10 3 10 316 16 16

10 1 316 16 1

16

x xdx dx dxx x x

x dx dxxx

+= +

+ + +

= ++ +

⌠ ⌠ ⌠ ⌡ ⌡ ⌡

⌠⌠ ⌡

⌡

We now have two integrals each requiring a different substitution. The substitutions for each of the integrals above are,

2 116 22

1 44 4

u x du xdx xdx du

xv dv dx dx dv

= + = ⇒ =

= = ⇒ =


( )

2 2

1

2 1

10 3 1 3 1516 4 1

35ln tan4

35ln 16 tan4 4

x dx du dvx u v

u v c

xx c

−

−

+= +

+ +

= + +

= + + +

⌠⌠ ⌠ ⌡ ⌡⌡

[Return to Problems] We’ve now seen a set of integrals in which we need to do more than one substitution. In these cases we will need to break up the integral into separate integrals and do separate substitutions for each. We now need to move onto a different set of examples that can be a little tricky. Once you’ve seen how to do these they aren’t too bad, but doing them for the first time can be difficult if you aren’t ready for them.


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Example 4 Evaluate each of the following integrals.

(a) tan x dx∫ [Solution]

(b) sec y dy∫ [Solution]

(c) ( )cos x

dxx

⌠⌡

[Solution]

(d) t tdt+∫ ee [Solution]

(e) 3 22 1x x dx+∫ [Solution] Solution

(a) tan x dx∫

The first question about this problem is probably why is it here? Substitution rule problems generally require more than a single function. The key to this problem is to realize that there really are two functions here. All we need to do is remember the definition of tangent and we can write the integral as,

sintancos

xx dx dxx

= ⌠⌡∫

Written in this way we can see that the following substitution will work for us, cos sin sinu x du x dx x dx du= = − ⇒ = − The integral is then,

1tan

ln

ln cos

x dx duuu c

x c

= −

= − +

= − +

⌠⌡∫

Now, while this is a perfectly serviceable answer that minus sign in front is liable to cause problems if we aren’t careful. So, let’s rewrite things a little. Recalling a property of logarithms we can move the minus sign in front to an exponent on the cosine and then do a little simplification.

1

tan ln cos

ln cos1ln

cos

ln sec

x dx x c

x c

cx

x c

−

= − +

= +

= +

= +

∫


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This is the formula that is typically given for the integral of tangent. Note that we could integrate cotangent in a similar manner.


(b) sec y dy∫

This problem also at first appears to not belong in the substitution rule problems. This is even more of a problem upon noticing that we can’t just use the definition of the secant function to write this in a form that will allow the use of the substitution rule. This problem is going to require a technique that isn’t used terribly often at this level, but is a useful technique to be aware of. Sometimes we can make an integral doable by multiplying the top and bottom by a common term. This will not always work and even when it does it is not always clear what we should multiply by but when it works it is very useful. Here is how we’ll use this idea for this problem.

( )( )sec tansecsec

1 sec tany yyy dy dyy y

+=

+⌠⌡

∫

First, we will think of the secant as a fraction and then multiply the top and bottom of the fraction by the same term. It is probably not clear why one would want to do this here but doing this will actually allow us to use the substitution rule. To see how this will work let’s simplify the integrand somewhat.

2sec tan secsecsec tan

y y yy dy dyy y+

=+

⌠⌡∫

We can now use the following substitution.

( )2sec tan sec tan secu y y du y y y dy= + = + The integral is then,

1sec

ln

ln sec tan

y dy duuu c

y y c

=

= +

= + +

⌠⌡∫

Sometimes multiplying the top and bottom of a fraction by a carefully chosen term will allow us to work a problem. It does however take some thought sometimes to determine just what the term should be. We can use a similar process for integrating cosecant.



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(c) ( )cos x

dxx

⌠⌡

This next problem has a subtlety to it that can get us in trouble if we aren’t paying attention. Because of the root in the cosine it makes some sense to use the following substitution.

1 12 21

2u x du x dx

−= =

This is where we need to be careful. Upon rewriting the differential we get,

12du dxx

=

The root that is in the denominator will not become a u as we might have been tempted to do. Instead it will get taken care of in the differential. The integral is,

( )( )

( )

( )

cos2 cos

2sin

2sin

xdx u du

x

u c

x c

=

= +

= +

⌠⌡

∫


(d) t tdt+∫ ee

With this problem we need to very carefully pick our substitution. As the problem is written we might be tempted to use the following substitution,

( )1t tu t du dt= + = +e e However, this won’t work as you can probably see. The differential doesn’t show up anywhere in the integrand and we just wouldn’t be able to eliminate all the t’s with this substitution. In order to work this problem we will need to rewrite the integrand as follows,

t tt tdt dt+ =∫ ∫e ee e e

We will now use the substitution, t tu du dt= =e e The integral is,


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t u

u

t

t

dt du

c

c

+ =

= +

= +

∫ ∫e

e

e e

e

e

Some substitutions can be really tricky to see and it’s not unusual that you’ll need to do some simplification and/or rewriting to get a substitution to work.


(e) 3 22 1x x dx+∫

This last problem in this set is different from all the other substitution problems that we’ve worked to this point. Given the fact that we’ve got more than an x under the root it makes sense that the substitution pretty much has to be, 2 1 2u x du x dx= + = At first glance it looks like this might not work for the substitution because we have an 3x in front of the root. However, if we first rewrite ( )3 22 2x x x= we could then move the 2x to the

end of the integral so at least the du will show up explicitly in the integral. Doing this gives the following,

( )3 2 2 2

12 2

2 1 1 2x x dx x x x dx

x u du

+ = +

=

∫ ∫

∫

This is a real problem. Our integrals can’t have two variables in them. Normally this would mean that we chose our substitution incorrectly. However, in this case we can rewrite the substitution as follows, 2 1x u= − and now, we can eliminate the remaining x’s from our integral. Doing this gives,

( )

( ) ( )

13 2 2

3 12 2

5 32 2

5 32 22 2

2 1 1

2 25 32 21 15 3

x x dx u u du

u u du

u u c

x x c

+ = −

= −

= − +

= + − + +

⌠⌡∫

∫

Sometimes, we will need to use a substitution more than once. This kind of problem doesn’t arise all that often and when it does there will sometimes be


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alternate methods of doing the integral. However, it will often work out that the easiest method of doing the integral is to do what we just did here.

[Return to Problems] This final set of examples isn’t too bad once you see the substitutions and that is the point with this set of problems. These all involve substitutions that we’ve not seen prior to this and so we need to see some of these kinds of problems. Example 5 Evaluate each of the following integrals.

(a) 1ln

dxx x

⌠⌡

[Solution]

(b) 2

21

t

t dt+

⌠⌡

ee

[Solution]

(c) 2

41

t

t dt+

⌠⌡

ee

[Solution]

(d) 1

2

sin1

x dxx

−

−

⌠⌡

[Solution]

Solution

(a) 1ln

dxx x

⌠⌡

In this case we know that we can’t integrate a logarithm by itself and so it makes some sense (hopefully) that the logarithm will need to be in the substitution. Here is the substitution for this problem.

1lnu x du dxx

= =

So the x in the denominator of the integrand will get substituted away with the differential. Here is the integral for this problem.

1 1ln

ln

ln ln

dx dux x u

u c

x c

=

= +

= +

⌠ ⌠ ⌡⌡


(b) 2

21

t

t dt+

⌠⌡

ee

Again, the substitution here may seem a little tricky. In this case the substitution is,


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2 2 2 11 22

t t tu du dt dt du= + = ⇒ =e e e


2

2

2

1 11 2

1 ln 12

t

t

t

dt duu

c

=+

= + +

⌠ ⌠ ⌡⌡e

e

e


(c) 2

41

t

t dt+

⌠⌡

ee

In this case we can’t use the same type of substitution that we used in the previous problem. In order to use the substitution in the previous example the exponential in the numerator and the denominator need to be the same and in this case they aren’t. To see the correct substitution for this problem note that,

( )24 2t t=e e

Using this, the integral can be written as follows,

( )

2 2

24 21 1

t t

t tdt dt=

+ +

⌠⌠ ⌡ ⌡

e ee e

We can now use the following substitution.

2 2 2 122

t t tu du dt dt du= = ⇒ =e e e


( )

( )

2

4 2

1

1 2

1 11 2 1

1 tan21 tan2

t

t

t

dt duu

u c

c

−

−

=+ +

= +

= +

⌠ ⌠ ⌡⌡e

e

e


(d) 1

2

sin1

x dxx

−

−

⌠⌡

This integral is similar to the first problem in this set. Since we don’t know how to integrate inverse sine functions it seems likely that this will be our substitution. If we use this as our


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substitution we get,

( )1

2

1sin1

u x du dxx

−= =−

So, the root in the integral will get taken care of in the substitution process and this will eliminate all the x’s from the integral. Therefore this was the correct substitution. The integral is,

( )

1

2

2

21

sin1

121 sin2

x dx u dux

u c

x c

−

−

=−

= +

= +

⌠⌡ ∫

[Return to Problems] Over the last couple of sections we’ve seen a lot of substitution rule examples. There are a couple of general rules that we will need to remember when doing these problems. First, when doing a substitution remember that when the substitution is done all the x’s in the integral (or whatever variable is being used for that particular integral) should all be substituted away. This includes the x in the dx. After the substitution only u’s should be left in the integral. Also, sometimes the correct substitution is a little tricky to find and more often than not there will need to be some manipulation of the differential or integrand in order to actually do the substitution. Also, many integrals will require us to break them up so we can do multiple substitutions so be on the lookout for those kinds of integrals/substitutions.


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Area Problem As noted in the first section of this section there are two kinds of integrals and to this point we’ve looked at indefinite integrals. It is now time to start thinking about the second kind of integral : Definite Integrals. However, before we do that we’re going to take a look at the Area Problem. The area problem is to definite integrals what the tangent and rate of change problems are to derivatives. The area problem will give us one of the interpretations of a definite integral and it will lead us to the definition of the definite integral. To start off we are going to assume that we’ve got a function ( )f x that is positive on some

interval [a,b]. What we want to do is determine the area of the region between the function and the x-axis. It’s probably easiest to see how we do this with an example. So let’s determine the area between

( ) 2 1f x x= + on [0,2]. In other words, we want to determine the area of the shaded region

below.

Now, at this point, we can’t do this exactly. However, we can estimate the area. We will estimate the area by dividing up the interval into n subintervals each of width,

b axn−

∆ =

Then in each interval we can form a rectangle whose height is given by the function value at a specific point in the interval. We can then find the area of each of these rectangles, add them up and this will be an estimate of the area. It’s probably easier to see this with a sketch of the situation. So, let’s divide up the interval into 4 subintervals and use the function value at the right endpoint of each interval to define the height of the rectangle. This gives,


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Note that by choosing the height as we did each of the rectangles will over estimate the area since each rectangle takes in more area than the graph each time. Now let’s estimate the area. First, the width of each of the rectangles is 1

2 . The height of each rectangle is determined by the function value at the right endpoint and so the height of each rectangle is nothing more that the function value at the right endpoint. Here is the estimated area.

( ) ( )

( ) ( )

1 1 1 1 3 11 22 2 2 2 2 21 5 1 1 13 12 52 4 2 2 4 25.75

rA f f f f = + + +

= + + +

=

Of course taking the rectangle heights to be the function value at the right endpoint is not our only option. We could have taken the rectangle heights to be the function value at the left endpoint. Using the left endpoints as the heights of the rectangles will give the following graph and estimated area.


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( ) ( )

( ) ( )

1 1 1 1 1 30 12 2 2 2 2 21 1 5 1 1 131 22 2 4 2 2 43.75

lA f f f f = + + +

= + + +

=

In this case we can see that the estimation will be an underestimation since each rectangle misses some of the area each time. There is one more common point for getting the heights of the rectangles that is often more accurate. Instead of using the right or left endpoints of each sub interval we could take the midpoint of each subinterval as the height of each rectangle. Here is the graph for this case.

So, it looks like each rectangle will over and under estimate the area. This means that the approximation this time should be much better than the previous two choices of points. Here is the estimation for this case.

1 1 1 3 1 5 1 72 4 2 4 2 4 2 41 17 1 25 1 41 1 652 16 2 16 2 16 2 164.625

mA f f f f = + + +

= + + +

=

We’ve now got three estimates. For comparison’s sake the exact area is

14 4.6663

A = =


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So, both the right and left endpoint estimation did not do all that great of a job at the estimation. The midpoint estimation however did quite well. Be careful to not draw any conclusion about how choosing each of the points will affect our estimation. In this case, because we are working with an increasing function choosing the right endpoints will overestimate and choosing left endpoint will underestimate. If we were to work with a decreasing function we would get the opposite results. For decreasing functions the right endpoints will underestimate and the left endpoints will overestimate. Also, if we had a function that both increased and decreased in the interval we would, in all likelihood, not even be able to determine if we would get an overestimation or underestimation. Now, let’s suppose that we want a better estimation, because none of the estimations above really did all that great of a job at estimating the area. We could try to find a different point to use for the height of each rectangle but that would be cumbersome and there wouldn’t be any guarantee that the estimation would in fact be better. Also, we would like a method for getting better approximations that would work for any function we would chose to work with and if we just pick new points that may not work for other functions. The easiest way to get a better approximation is to take more rectangles (i.e. increase n). Let’s double the number of rectangles that we used and see what happens. Here are the graphs showing the eight rectangles and the estimations for each of the three choices for rectangle heights that we used above.


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Here are the area estimations for each of these cases. 5.1875 4.1875 4.65625r l mA A A= = = So, increasing the number of rectangles did improve the accuracy of the estimation as we’d guessed that it would. Let’s work a slightly more complicated example. Example 1 Estimate the area between ( ) 3 25 6 5f x x x x= − + + and the x-axis using 5n = subintervals and all three cases above for the heights of each rectangle. Solution First, let’s get the graph to make sure that the function is positive.

So, the graph is positive and the width of each subinterval will be,

4 0.85

x∆ = =

This means that the endpoints of the subintervals are, 0, 0.8, 1.6, 2.4, 3.2, 4 Let’s first look at using the right endpoints for the function height. Here is the graph for this case.


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Notice, that unlike the first area we looked at, the choosing the right endpoints here will both over and underestimate the area depending on where we are on the curve. This will often be the case with a more general curve that the one we initially looked at. The area estimation using the right endpoints of each interval for the rectangle height is,

( ) ( ) ( ) ( ) ( )0.8 0.8 0.8 1.6 0.8 2.4 0.8 3.2 0.8 4

28.96rA f f f f f= + + + +

=

Now let’s take a look at left endpoints for the function height. Here is the graph.

The area estimation using the left endpoints of each interval for the rectangle height is,

( ) ( ) ( ) ( ) ( )0.8 0 0.8 0.8 0.8 1.6 0.8 2.4 0.8 3.2

22.56rA f f f f f= + + + +

=

Finally, let’s take a look at the midpoints for the heights of each rectangle. Here is the graph,


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The area estimation using the midpoint is then,

( ) ( ) ( ) ( ) ( )0.8 0.4 0.8 1.2 0.8 2 0.8 2.8 0.8 3.6

25.12rA f f f f f= + + + +

=

For comparison purposes the exact area is,

76 25.3333

A = =

So, again the midpoint did a better job than the other two. While this will be the case more often than not, it won’t always be the case and so don’t expect this to always happen. Now, let’s move on to the general case. Let’s start out with ( ) 0f x ≥ on [a,b] and we’ll divide

the interval into n subintervals each of length,

b axn−

∆ =

Note that the subintervals don’t have to be equal length, but it will make our work significantly easier. The endpoints of each subinterval are,

( )

0

1

2

1

2

1

i

n

n

x ax a xx a x

x a i x

x a n xx a n x b

−

== + ∆= + ∆

= + ∆

= + − ∆

= + ∆ =

M

M


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Next in each interval,

[ ] [ ] [ ] [ ]0 1 1 2 1 1, , , , , , , , ,i i n nx x x x x x x x− −… …

we choose a point * * * *1 2, , , ,i nx x x x… … . These points will define the height of the rectangle in

each subinterval. Note as well that these points do not have to occur at the same point in each subinterval. Here is a sketch of this situation.

The area under the curve on the given interval is then approximately,

( ) ( ) ( ) ( )* * * *1 2 i nA f x x f x x f x x f x x≈ ∆ + ∆ + + ∆ + + ∆L L

We will use summation notation or sigma notation at this point to simplify up our notation a little. If you need a refresher on summation notation check out the section devoted to this in the Extras chapter. Using summation notation the area estimation is,

( )*

1

n

ii

A f x x=

≈ ∆∑

The summation in the above equation is called a Riemann Sum. To get a better estimation we will take n larger and larger. In fact, if we let n go out to infinity we will get the exact area. In other words,

( )*

1lim

n

in iA f x x

→∞=

= ∆∑

Before leaving this section let’s address one more issue. To this point we’ve required the function to be positive in our work. Many functions are not positive however. Consider the case


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of ( ) 2 4f x x= − on [0,2]. If we use 8n = and the midpoints for the rectangle height we get the

following graph,

In this case let’s notice that the function lies completely below the x-axis and hence is always negative. If we ignore the fact that the function is always negative and use the same ideas above to estimate the area between the graph and the x-axis we get,

1 1 1 3 1 5 1 7 1 94 8 4 8 4 8 4 8 4 8

1 11 1 13 1 154 8 4 8 4 8

5.34375

mA f f f f f

f f f

= + + + + +

+ +

= −

Our answer is negative as we might have expected given that all the function evaluations are negative. So, using the technique in this section it looks like if the function is above the x-axis we will get a positive area and if the function is below the x-axis we will get a negative area. Now, what about a function that is both positive and negative in the interval? For example, ( ) 2 2f x x= − on

[0,2]. Using 8n = and midpoints the graph is,


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Some of the rectangles are below the x-axis and so will give negative areas while some are above the x-axis and will give positive areas. Since more rectangles are below the x-axis than above it looks like we should probably get a negative area estimation for this case. In fact that is correct. Here the area estimation for this case.

1 1 1 3 1 5 1 7 1 94 8 4 8 4 8 4 8 4 8

1 11 1 13 1 154 8 4 8 4 8

1.34375

mA f f f f f

f f f

= + + + +

+ +

= −

In cases where the function is both above and below the x-axis the technique given in the section will give the net area between the function and the x-axis with areas below the x-axis negative and areas above the x-axis positive. So, if the net area is negative then there is more area under the x-axis than above while a positive net area will mean that more of the area is above the x-axis.


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The Definition of the Definite Integral In this section we will formally define the definite integral and give many of the properties of definite integrals. Let’s start off with the definition of a definite integral. Definite Integral Given a function ( )f x that is continuous on the interval [a,b] we divide the interval into n

subintervals of equal width, x∆ , and from each interval choose a point, *ix . Then the definite

integral of f(x) from a to b is

( ) ( )*

1lim

nb

ia n if x dx f x x

→∞=

= ∆∑∫

The definite integral is defined to be exactly the limit and summation that we looked at in the last section to find the net area between a function and the x-axis. Also note that the notation for the definite integral is very similar to the notation for an indefinite integral. The reason for this will be apparent eventually. There is also a little bit of terminology that we should get out of the way here. The number “a” that is at the bottom of the integral sign is called the lower limit of the integral and the number “b” at the top of the integral sign is called the upper limit of the integral. Also, despite the fact that a and b were given as an interval the lower limit does not necessarily need to be smaller than the upper limit. Collectively we’ll often call a and b the interval of integration. Let’s work a quick example. This example will use many of the properties and facts from the brief review of summation notation in the Extras chapter. Example 1 Using the definition of the definite integral compute the following.

2 2

01x dx+∫

Solution First, we can’t actually use the definition unless we determine which points in each interval that well use for *

ix . In order to make our life easier we’ll use the right endpoints of each interval. From the previous section we know that for a general n the width of each subinterval is,

2 0 2xn n−

∆ = =

The subintervals are then,

( ) ( )2 1 2 12 2 4 4 6 20, , , , , , , , , , , 2i ni

n n n n n n n n− −

… …

As we can see the right endpoint of the ith subinterval is


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* 2i

ixn

=

The summation in the definition of the definite integral is then,

( )*

1 1

2

1

2

31

2 2

2 21

8 2

n n

ii i

n

i

n

i

if x x fn n

in n

in n

= =

=

=

∆ =

= +

= +

∑ ∑

∑

∑

Now, we are going to have to take a limit of this. That means that we are going to need to “evaluate” this summation. In other words, we are going to have to use the formulas given in the summation notation review to eliminate the actual summation and get a formula for this for a general n. To do this we will need to recognize that n is a constant as far as the summation notation is concerned. As we cycle through the integers from 1 to n in the summation only i changes and so anything that isn’t an i will be a constant and can be factored out of the summation. In particular any n that is in the summation can be factored out if we need to. Here is the summation “evaluation”.

( )

( ) ( ) ( )

( ) ( )

2*

31 1 1

23

1 1

3

2

2

2

8 2

8 1 2

1 2 18 1 26

4 1 2 12

314 12 4

3

n n n

ii i i

n n

i i

if x xn n

in n

n n nn

n n

n nn

n nn

= = =

= =

∆ = +

= +

+ + = +

+ +

= +

+ +=

∑ ∑ ∑

∑ ∑

We can now compute the definite integral.


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( )2 2 *

01

2

2

1 lim

14 12 4lim3

143

n

in i

n

x dx f x x

n nn

→∞=

→∞

+ = ∆

+ +=

=

∑∫

We’ve seen several methods for dealing with the limit in this problem so I’ll leave it to you to verify the results. Wow, that was a lot of work for a fairly simple function. There is a much simpler way of evaluating these and we will get to it eventually. The main purpose to this section is to get the main properties and facts about the definite integral out of the way. We’ll discuss how we compute these in practice starting with the next section. So, let’s start taking a look at some of the properties of the definite integral. Properties

1. ( ) ( )b a

a bf x dx f x dx= −∫ ∫ . We can interchange the limits on any definite integral, all that

we need to do is tack a minus sign onto the integral when we do.

2. ( ) 0a

af x dx =∫ . If the upper and lower limits are the same then there is no work to do, the

integral is zero.

3. ( ) ( )b b

a acf x dx c f x dx=∫ ∫ , where c is any number. So, as with limits, derivatives, and

indefinite integrals we can factor out a constant.

4. ( ) ( ) ( ) ( )b b b

a a af x g x dx f x dx g x dx± = ±∫ ∫ ∫ . We can break up definite integrals across a

sum or difference.

5. ( ) ( ) ( )b c b

a a cf x dx f x dx f x dx= +∫ ∫ ∫ where c is any number. This property is more

important than we might realize at first. One of the main uses of this property is to tell us how we can integrate a function over the adjacent intervals, [a,c] and [c,b]. Note however that c doesn’t need to be between a and b.

6. ( ) ( )b b

a af x dx f t dt=∫ ∫ . The point of this property is to notice that as long as the function

and limits are the same the variable of integration that we use in the definite integral won’t affect the answer.


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See the Proof of Various Integral Properties section of the Extras chapter for the proof of properties 1 – 4. Property 5 is not easy to prove and so is not shown there. Property 6 is not really a property in the full sense of the word. It is only here to acknowledge that as long as the function and limits are the same it doesn’t matter what letter we use for the variable. The answer will be the same. Let’s do a couple of examples dealing with these properties. Example 2 Use the results from the first example to evaluate each of the following.

(a) 0 2

21x dx+∫ [Solution]

(b) 2 2

010 10x dx+∫ [Solution]

(c) 2 2

01t dt+∫ [Solution]

Solution All of the solutions to these problems will rely on the fact we proved in the first example. Namely that,

2 2

0

1413

x dx+ =∫

(a) 0 2

21x dx+∫

In this case the only difference between the two is that the limits have interchanged. So, using the first property gives,

0 22 2

2 01 1

143

x dx x dx+ = − +

= −

∫ ∫


(b) 2 2

010 10x dx+∫

For this part notice that we can factor a 10 out of both terms and then out of the integral using the third property.

( )2 22 2

0 0

2 2

0

10 10 10 1

10 1

14103

1403

x dx x dx

x dx

+ = +

= +

=

=

∫ ∫

∫



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(c) 2 2

01t dt+∫

In this case the only difference is the letter used and so this is just going to use property 6.

2 22 2

0 0

141 13

t dt x dx+ = + =∫ ∫

[Return to Problems] Here are a couple of examples using the other properties. Example 3 Evaluate the following definite integral.

( ) ( )130 3

2130

sin cos1

x x x xdx

x− +

+⌠⌡

Solution There really isn’t anything to do with this integral once we notice that the limits are the same. Using the second property this is,

( ) ( )130 3

2130

sin cos0

1x x x x

dxx

− +=

+⌠⌡

Example 4 Given that ( )10

623f x dx

−=∫ and ( )

6

109g x dx

−= −∫ determine the value of

( ) ( )6

102 10f x g x dx

−−∫

Solution We will first need to use the fourth property to break up the integral and the third property to factor out the constants.

( ) ( ) ( ) ( )

( ) ( )

6 6 6

10 10 10

6 6

10 10

2 10 2 10

2 10

f x g x dx f x dx g x dx

f x dx g x dx

− − −

− −

− = −

= −

∫ ∫ ∫

∫ ∫

Now notice that the limits on the first integral are interchanged with the limits on the given integral so switch them using the first property above (and adding a minus sign of course). Once this is done we can plug in the known values of the integrals.

( ) ( ) ( ) ( )

( ) ( )

6 10 6

10 6 102 10 2 10

2 23 10 944

f x g x dx f x dx g x dx−

− −− = − −

= − − −

=

∫ ∫ ∫


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Example 5 Given that ( )10

126f x dx

−=∫ , ( )

10

1002f x dx

−= −∫ , and ( )

5

1004f x dx

−=∫ determine

the value of ( )12

5f x dx

−∫ .

Solution This example is mostly an example of property 5 although there are a couple of uses of property 1 in the solution as well. We need to figure out how to correctly break up the integral using property 5 to allow us to use the given pieces of information. First we’ll note that there is an integral that has a “-5” in one of the limits. It’s not the lower limit, but we can use property 1 to correct that eventually. The other limit is 100 so this is the number c that we’ll use in property 5.

( ) ( ) ( )12 100 12

5 5 100f x dx f x dx f x dx

− −= +∫ ∫ ∫

We’ll be able to get the value of the first integral, but the second still isn’t in the list of know integrals. However, we do have second limit that has a limit of 100 in it. The other limit for this second integral is -10 and this will be c in this application of property 5.

( ) ( ) ( ) ( )12 100 10 12

5 5 100 10f x dx f x dx f x dx f x dx

−

− − −= + +∫ ∫ ∫ ∫

At this point all that we need to do is use the property 1 on the first and third integral to get the limits to match up with the known integrals. After that we can plug in for the known integrals.

( ) ( ) ( ) ( )12 5 10 10

5 100 100 12

4 2 612

f x dx f x dx f x dx f x dx− − −

−= − + −

= − − −= −

∫ ∫ ∫ ∫

There are also some nice properties that we can use in comparing the general size of definite integrals. Here they are. More Properties

7. ( )b

ac dx c b a= −∫ , c is any number.

8. If ( ) 0f x ≥ for a x b≤ ≤ then ( ) 0b

af x dx ≥∫ .

9. If ( ) ( )f x g x≥ for a x b≤ ≤ then ( ) ( )b b

a af x dx g x dx≥∫ ∫ .

10. If ( )m f x M≤ ≤ for a x b≤ ≤ then ( ) ( ) ( )b

am b a f x dx M b a− ≤ ≤ −∫ .

11. ( ) ( )b b

a af x dx f x dx≤∫ ∫


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See the Proof of Various Integral Properties section of the Extras chapter for the proof of these properties. Interpretations of Definite Integral There are a couple of quick interpretations of the definite integral that we can give here. First, as we alluded to in the previous section one possible interpretation of the definite integral is to give the net area between the graph of ( )f x and the x-axis on the interval [a,b]. So, the net

area between the graph of ( ) 2 1f x x= + and the x-axis on [0,2] is,

2 2

0

1413

x dx+ =∫

If you look back in the last section this was the exact area that was given for the initial set of problems that we looked at in this area. Another interpretation is sometimes called the Net Change Theorem. This interpretation says that if ( )f x is some quantity (so ( )f x′ is the rate of change of ( )f x , then,

( ) ( ) ( )b

af x dx f b f a′ = −∫

is the net change in ( )f x on the interval [a,b]. In other words, compute the definite integral of

a rate of change and you’ll get the net change in the quantity. We can see that the value of the definite integral, ( ) ( )f b f a− , does in fact give use the net change in ( )f x and so there really

isn’t anything to prove with this statement. This is really just an acknowledgment of what the definite integral of a rate of change tells us. So as a quick example, if ( )V t is the volume of water in a tank then,

( ) ( ) ( )2

12 1

t

tV t dt V t V t′ = −∫

is the net change in the volume as we go from time 1t to time 2t . Likewise, if ( )s t is the function giving the position of some object at time t we know that the

velocity of the object at any time t is : ( ) ( )v t s t′= . Therefore the displacement of the object

time 1t to time 2t is,

( ) ( ) ( )2

12 1

t

tv t dt s t s t= −∫

Note that in this case if ( )v t is both positive and negative (i.e. the object moves to both the right

and left) in the time frame this will NOT give the total distance traveled. It will only give the displacement, i.e. the difference between where the object started and where it ended up. To get the total distance traveled by an object we’d have to compute,


Math 140


( )2

1

t

tv t dt∫

It is important to note here that the Net Change Theorem only really makes sense if we’re integrating a derivative of a function. Fundamental Theorem of Calculus, Part I As noted by the title above this is only the first part to the Fundamental Theorem of Calculus. We will give the second part in the next section as it is the key to easily computing definite integrals and that is the subject of the next section. The first part of the Fundamental Theorem of Calculus tells us how to differentiate certain types of definite integrals and it also tells us about the very close relationship between integrals and derivatives. Fundamental Theorem of Calculus, Part I If ( )f x is continuous on [a,b] then,

( ) ( )x

ag x f t dt= ∫

is continuous on [a,b] and it is differentiable on ( ),a b and that,

( ) ( )g x f x′ = An alternate notation for the derivative portion of this is,

( ) ( )x

a

d f t dt f xdx

=∫

To see the proof of this see the Proof of Various Integral Properties section of the Extras chapter. Let’s check out a couple of quick examples using this. Example 6 Differentiate each of the following.

(a) ( ) ( )2 2

4cos 1 5

x tg x t dt−

= −∫ e [Solution]

(b) 2

1 4

2

11x

t dtt

++

⌠⌡

[Solution]

Solution

(a) ( ) ( )2 2

4cos 1 5

x tg x t dt−

= −∫ e

This one is nothing more than a quick application of the Fundamental Theorem of Calculus. ( ) ( )2 2cos 1 5xg x x′ = −e



Math 140


(b) 2

1 4

2

11x

t dtt

++

⌠⌡

This one needs a little work before we can use the Fundamental Theorem of Calculus. The first thing to notice is that the FToC requires the lower limit to be a constant and the upper limit to be the variable. So, using a property of definite integrals we can interchange the limits of the integral we just need to remember to add in a minus sign after we do that. Doing this gives,

2 2

2

1 4 4 4

2 2 21 1

1 1 11 1 1

x x

x

d t d t d tdt dt dtdx t dx t dx t

+ + += − = − + + +

⌠ ⌠ ⌠ ⌡ ⌡ ⌡

The next thing to notice is that the FToC also requires an x in the upper limit of integration and we’ve got x2. To do this derivative we’re going to need the following version of the chain rule.

( )( ) ( )( ) ( )where d d dug u g u u f xdx du dx

= =

So, if we let u= x2 we use the chain rule to get,

( )

2

2

1 4 4

2 21

42

21

4

2

4

2

1 11 1

1 where 1

1 21

121

x

x

u

d t d tdt dtdx t dx t

d t dudt u xdu t dx

u xu

uxu

+ += −

+ +

+= − =

+

+= −

++

= −+

⌠ ⌠ ⌡ ⌡

⌠⌡

The final step is to get everything back in terms of x.

( )( )2

421 4

22 2

8

4

11 21 1

121

x

xd t dt xdx t x

xxx

++= −

+ +

+= −

+

⌠⌡

[Return to Problems] Using the chain rule as we did in the last part of this example we can derive some general formulas for some more complicated problems. First,

( )( ) ( ) ( )( )u x

a

d f t dt u x f u xdx

′=∫

This is simply the chain rule for these kinds of problems.


Math 140


Next, we can get a formula for integrals in which the upper limit is a constant and the lower limit is a function of x. All we need to do here is interchange the limits on the integral (adding in a minus sign of course) and then using the formula above to get,

( )( )

( )( ) ( ) ( )( )b v x

v x b

d df t dt f t dt v x f v xdx dx

′= − = −∫ ∫

Finally, we can also get a version for both limits being functions of x. In this case we’ll need to use Property 5 above to break up the integral as follows,

( )( )

( ) ( )( )

( )( )u x a u x

v x v x af t dt f t dt f t dt= +∫ ∫ ∫

We can use pretty much any value of a when we break up the integral. The only thing that we need to avoid is to make sure that ( )f a exists. So, assuming that ( )f a exists after we break

up the integral we can then differentiate and use the two formulas above to get,

( )

( )

( ) ( )( )

( )( )( )( ) ( )( ) ( ) ( )( )

u x a u x

v x v x a

d df t dt f t dt f t dtdx dx

v x f v x u x f u x

= +

′ ′= − +

∫ ∫ ∫

Let’s work a quick example. Example 7 Differentiate the following integral.

( )3 2 2sin 1x

xt t dt+∫

Solution This will use the final formula that we derived above.

( ) ( ) ( )( ) ( ) ( ) ( )( )

( ) ( )

1 2 23 2 22 2 2

2 2

1sin 1 sin 1 3 3 sin 1 321 sin 1 27 sin 1 92

x

x

d t t dt x x x x xdx

x x x x

−+ = − + + +

= − + + +

∫


Math 140


Computing Definite Integrals In this section we are going to concentrate on how we actually evaluate definite integrals in practice. To do this we will need the Fundamental Theorem of Calculus, Part II. Fundamental Theorem of Calculus, Part II Suppose ( )f x is a continuous function on [a,b] and also suppose that ( )F x is any anti-

derivative for ( )f x . Then,

( ) ( ) ( ) ( )b b

aaf x dx F x F b F a= = −∫

To see the proof of this see the Proof of Various Integral Properties section of the Extras chapter. Recall that when we talk about an anti-derivative for a function we are really talking about the indefinite integral for the function. So, to evaluate a definite integral the first thing that we’re going to do is evaluate the indefinite integral for the function. This should explain the similarity in the notations for the indefinite and definite integrals. Also notice that we require the function to be continuous in the interval of integration. This was also a requirement in the definition of the definite integral. We didn’t make a big deal about this in the last section. In this section however, we will need to keep this condition in mind as we do our evaluations. Next let’s address the fact that we can use any anti-derivative of ( )f x in the evaluation. Let’s

take a final look at the following integral.

2 2

01x dx+∫

Both of the following are anti-derivatives of the integrand.

( ) ( )3 31 1 18and3 3 31

F x x x F x x x= + = + −

Using the FToC to evaluate this integral with the first anti-derivatives gives,

( ) ( )

22 2 3

00

3 3

113

1 12 2 0 03 3143

x dx x x + = +

= + − +

=

∫


Math 140


Much easier than using the definition wasn’t it? Let’s now use the second anti-derivative to evaluate this definite integral.

( ) ( )

22 2 3

00

3 3

1 1813 31

1 18 1 182 2 0 03 31 3 3114 18 183 31 31

143

x dx x x + = + −

= + − − + −

= − +

=

∫

The constant that we tacked onto the second anti-derivative canceled in the evaluation step. So, when choosing the anti-derivative to use in the evaluation process make your life easier and don’t bother with the constant as it will only end up canceling in the long run. Also, note that we’re going to have to be very careful with minus signs and parenthesis with these problems. It’s very easy to get in a hurry and mess them up. Let’s start our examples with the following set designed to make a couple of quick points that are very important. Example 1 Evaluate each of the following.

(a) 2 2y y dy−+∫ [Solution]

(b) 2 2 2

1y y dy−+∫ [Solution]

(c) 2 2 2

1y y dy−

−+∫ [Solution]

Solution

(a) 2 2y y dy−+∫

This is the only indefinite integral in this section and by now we should be getting pretty good with these so we won’t spend a lot of time on this part. This is here only to make sure that we understand the difference between an indefinite and a definite integral. The integral is,

2 2 3 113

y y dy y y c− −+ = − +∫


(b) 2 2 2

1y y dy−+∫

Recall from our first example above that all we really need here is any anti-derivative of the integrand. We just computed the most general anti-derivative in the first part so we can use that if we want to. However, recall that as we noted above any constants we tack on will just cancel


Math 140


in the long run and so we’ll use the answer from (a) without the “+c”. Here’s the integral,

( ) ( )

22 2 2 3

11

3 3

1 13

1 1 1 12 13 2 3 18 1 1 13 2 3176

y y dy yy

− + = −

= − − −

= − − +

=

∫

Remember that the evaluation is always done in the order of evaluation at the upper limit minus evaluation at the lower limit. Also be very careful with minus signs and parenthesis. It’s very easy to forget them or mishandle them and get the wrong answer. Notice as well that, in order to help with the evaluation, we rewrote the indefinite integral a little. In particular we got rid of the negative exponent on the second term. It’s generally easier to evaluate the term with positive exponents.


(c) 2 2 2

1y y dy−

−+∫

This integral is here to make a point. Recall that in order for us to do an integral the integrand must be continuous in the range of the limits. In this case the second term will have division by zero at 0y = and since 0y = is in the interval of integration, i.e. it is between the lower and upper limit, this integrand is not continuous in the interval of integration and so we can’t do this integral. Note that this problem will not prevent us from doing the integral in (b) since 0y = is not in the interval of integration.

[Return to Problems] So what have we learned from this example? First, in order to do a definite integral the first thing that we need to do is the indefinite integral. So we aren’t going to get out of doing indefinite integrals, they will be in every integral that we’ll be doing in the rest of this course so make sure that you’re getting good at computing them. Second, we need to be on the lookout for functions that aren’t continuous at any point between the limits of integration. Also, it’s important to note that this will only be a problem if the


Math 140


point(s) of discontinuity occur between the limits of integration or at the limits themselves. If the point of discontinuity occurs outside of the limits of integration the integral can still be evaluated. In the following sets of examples we won’t make too much of an issue with continuity problems, or lack of continuity problems, unless it affects the evaluation of the integral. Do not let this convince you that you don’t need to worry about this idea. It arises often enough that it can cause real problems if you aren’t on the lookout for it. Finally, note the difference between indefinite and definite integrals. Indefinite integrals are functions while definite integrals are numbers. Let’s work some more examples. Example 2 Evaluate each of the following.

(a) 1 2

36 5 2x x dx

−− +∫ [Solution]

(b) ( )0

42t t dt−∫ [Solution]

(c) 2 5

21

2 3w w dww− +⌠

⌡

[Solution]

(d) 10

25dR

−

∫ [Solution]

Solution

(a) 1 2

36 5 2x x dx

−− +∫

There isn’t a lot to this one other than simply doing the work.

11 2 3 2

33

56 5 2 2 22

5 452 2 54 62 2

84

x x dx x x x−

−

− + = − +

= − + − − − −

=

∫


(b) ( )0

42t t dt−∫

Recall that we can’t integrate products as a product of integrals and so we first need to multiply the integrand out before integrating, just as we did in the indefinite integral case.


Math 140


( )

( ) ( )

3 10 02 2

4 4

05 32 2

4

5 32 2

2 2

2 45 3

2 40 4 45 3

3215

t t dt t t dt

t t

− = −

= −

= − −

= −

∫ ∫

In the evaluation process recall that,

( ) ( ) ( )55 1 5

2 24 4 2 32 = = =

( ) ( ) ( )33 1 3

2 24 4 2 8 = = =

Also, don’t get excited about the fact that the lower limit of integration is larger than the upper limit of integration. That will happen on occasion and there is absolutely nothing wrong with this.


(c) 2 5

21

2 3w w dww− +⌠

⌡

First, notice that we will have a division by zero issue at 0w = , but since this isn’t in the interval of integration we won’t have to worry about it. Next again recall that we can’t integrate quotients as a quotient of integrals and so the first step that we’ll need to do is break up the quotient so we can integrate the function.

2 253 2

211

24

1

2 3 12 3

1 3ln2

3 18 ln 2 ln1 32 2

9 ln 2

w w dw w w dww w

w ww

−− += − +

= − −

= − − − − −

= −

⌠ ⌠ ⌡⌡

Don’t get excited about answers that don’t come down to a simple integer or fraction. Often times they won’t. Also don’t forget that ( )ln 1 0= .



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(d) 10

25dR

−

∫

This one is actually pretty easy. Recall that we’re just integrating 1!.

10 10

2525

10 2535

dR R− −=

= − −= −

∫

[Return to Problems] The last set of examples dealt exclusively with integrating powers of x. Let’s work a couple of examples that involve other functions. Example 3 Evaluate each of the following.

(a) 1 3 2

04 6x x dx−∫ [Solution]

(b) 30

2sin 5cos dπ

θ θ θ−∫ [Solution]

(c) 4

65 2sec tanz z dz

π

π−∫ [Solution]

(d) 1

20

3 13z dz

z

−

−−

−⌠⌡ e

[Solution]

(e) 3

6

2

15 10t t dtt−

− +⌠⌡

[Solution]

Solution

(a) 1 3 2

04 6x x dx−∫ .

This one is here mostly here to contrast with the next example.

( )

21 13 2 30 0

152 3

0

4 6 4 6

1825

182 05

85

x x dx x x dx

x x

− = −

= −

= − −

= −

∫ ∫


(b) 30

2sin 5cos dπ

θ θ θ−∫

Be careful with signs with this one. Recall from the indefinite integral sections that it’s easy to mess up the signs when integrating sine and cosine.


Math 140


( )

( )

3300

2sin 5cos 2cos 5sin

2cos 5sin 2cos 0 5sin 03 3

5 31 22

5 312

dπ

πθ θ θ θ θ

π π

− = − −

= − − − − −

= − − +

= −

∫

Compare this answer to the previous answer, especially the evaluation at zero. It’s very easy to get into the habit of just writing down zero when evaluating a function at zero. This is especially a problem when many of the functions that we integrate involve only x’s raised to positive integers and in these cases evaluate is zero of course. After evaluating many of these kinds of definite integrals it’s easy to get into the habit of just writing down zero when you evaluate at zero. However, there are many functions out there that aren’t zero when evaluated at zero so be careful.


(c) 4

65 2sec tanz z dz

π

π−∫

Not much to do other than do the integral.

( )4 4

665 2sec tan 5 2sec

5 2sec 5 2sec4 4 6 6

5 42 212 3

z z dz z zπ π

ππ

π π π π

π

− = −

= − − −

= − +

∫

For the evaluation, recall that

1seccos

zz

=

and so if we can evaluate cosine at these angles we can evaluate secant at these angles. [Return to Problems]

(d) 1

20

3 13z dz

z

−

−−

−⌠⌡ e

In order to do this one will need to rewrite both of the terms in the integral a little as follows,

1 1

20 20

3 1 1 133 3

zz dz dz

z z

− −

−− −

− = −⌠ ⌠ ⌡ ⌡

ee

For the first term recall we used the following fact about exponents.


Math 140


1 1a aa ax x

x x−

−= =

In the second term, taking the 3 out of the denominator will just make integrating that term easier. Now the integral.

11

20 20

1 20

1 20

3 1 13 ln3 3

1 13 ln 1 3 ln 203 3

13 3 ln 203

zz dz z

z

−−

−− −

− −

− −

− = −

= − − − − −

= − +

⌠⌡

ee

e e

e e

Just leave the answer like this. It’s messy, but it’s also exact. Note that the absolute value bars on the logarithm are required here. Without them we couldn’t have done the evaluation.


(e) 3

6

2

15 10t t dtt−

− +⌠⌡

This integral can’t be done. There is division by zero in the third term at 0t = and 0t = lies in the interval of integration. The fact that the first two terms can be integrated doesn’t matter. If even one term in the integral can’t be integrated then the whole integral can’t be done.

[Return to Problems] So, we’ve computed a fair number of definite integrals at this point. Remember that the vast majority of the work in computing them is first finding the indefinite integral. Once we’ve found that the rest is just some number crunching. There are a couple of particularly tricky definite integrals that we need to take a look at next. Actually they are only tricky until you see how to do them, so don’t get too excited about them. The first one involves integrating a piecewise function. Example 4 Given,

( ) 2

6 if 13 if 1

xf x

x x>

= ≤

Evaluate each of the following integrals.

(a) ( )22

10f x dx∫ [Solution]

(b) ( )3

2f x dx

−∫ [Solution]

Solution Let’s first start with a graph of this function.


Math 140


The graph reveals a problem. This function is not continuous at 1x = and we’re going to have to watch out for that.

(a) ( )22

10f x dx∫

For this integral notice that 1x = is not in the interval of integration and so that is something that we’ll not need to worry about in this part. Also note the limits for the integral lie entirely in the range for the first function. What this means for us is that when we do the integral all we need to do is plug in the first function into the integral. Here is the integral.

( )22 22

10 10

22

10

6

6

132 6072

f x dx dx

x

=

=

= −=

∫ ∫


(b) ( )3

2f x dx

−∫

In this part 1x = is between the limits of integration. This means that the integrand is no longer continuous in the interval of integration and that is a show stopper as far we’re concerned. As noted above we simply can’t integrate functions that aren’t continuous in the interval of integration. Also, even if the function was continuous at 1x = we would still have the problem that the function is actually two different equations depending where we are in the interval of integration.


Math 140


Let’s first address the problem of the function not beginning continuous at 1x = . As we’ll see, in this case, if we can find a way around this problem the second problem will also get taken care of at the same time. In the previous examples where we had functions that weren’t continuous we had division by zero and no matter how hard we try we can’t get rid of that problem. Division by zero is a real problem and we can’t really avoid it. In this case the discontinuity does not stem from problems with the function not existing at 1x = . Instead the function is not continuous because it takes on different values on either sides of 1x = . We can “remove” this problem by recalling Property 5 from the previous section. This property tells us that we can write the integral as follows,

( ) ( ) ( )3 1 3

2 2 1f x dx f x dx f x dx

− −= +∫ ∫ ∫

On each of these intervals the function is continuous. In fact we can say more. In the first integral we will have x between -2 and 1 and this means that we can use the second equation for

( )f x and likewise for the second integral x will be between 1 and 3 and so we can use the first

function for ( )f x . The integral in this case is then,

( ) ( ) ( )

( ) ( )

3 1 3

2 2 1

1 32

2 1

1 3312

3 6

6

1 8 18 621

f x dx f x dx f x dx

x dx dx

x x

− −

−

−

= +

= +

= +

= − − + −

=

∫ ∫ ∫

∫ ∫


So, to integrate a piecewise function, all we need to do is break up the integral at the break point(s) that happen to occur in the interval of integration and then integrate each piece. Next we need to look at is how to integrate an absolute value function. Example 5 Evaluate the following integral.

3

03 5t dt−∫

Solution Recall that the point behind indefinite integration (which we’ll need to do in this problem) is to determine what function we differentiated to get the integrand. To this point we’ve not seen any functions that will differentiate to get an absolute value nor will we ever see a function that will differentiate to get an absolute value.


Math 140


The only way that we can do this problem is to get rid of the absolute value. To do this we need to recall the definition of absolute value.

if 0if 0

x xx

x x≥

= − <

Once we remember that we can define absolute value as a piecewise function we can use the work from Example 4 as a guide for doing this integral. What we need to do is determine where the quantity on the inside of the absolute value bars is negative and where it is positive. It looks like if 5

3t > the quantity inside the absolute value is

positive and if 53t < the quantity inside the absolute value is negative.

Next, note that 5

3t = is in the interval of integration and so, if we break up the integral at this point we get,

53 33

50 03

3 5 3 5 3 5t dt t dt t dt− = − + −∫ ∫ ∫

Now, in the first integrals we have 5

3t < and so 3 5 0t − < in this interval of integration. That means we can drop the absolute value bars if we put in a minus sign. Likewise in the second integral we have 5

3t > which means that in this interval of integration we have 3 5 0t − > and so we can just drop the absolute value bars in this integral. After getting rid of the absolute value bars in each integral we can do each integral. So, doing the integration gives,

( )

( ) ( ) ( )

53 33

50 03

5 33

503

5 33

2 2

503

2 22

3 5 3 5 3 5

3 5 3 5

3 35 52 2

3 5 5 3 3 5 55 0 3 5 3 52 3 3 2 2 3 3

25 86 3416

t dt t dt t dt

t dt t dt

t t t t

− = − − + −

= − + + −

= − + + −

= − + − + − − −

= +

=

∫ ∫ ∫

∫ ∫


Math 140


Integrating absolute value functions isn’t too bad. It’s a little more work than the “standard” definite integral, but it’s not really all that much more work. First, determine where the quantity inside the absolute value bars is negative and where it is positive. When we’ve determined that point all we need to do is break up the integral so that in each range of limits the quantity inside the absolute value bars is always positive or always negative. Once this is done we can drop the absolute value bars (adding negative signs when the quantity is negative) and then we can do the integral as we’ve always done.


Math 140


Substitution Rule for Definite Integrals We now need to go back and revisit the substitution rule as it applies to definite integrals. At some level there really isn’t a lot to do in this section. Recall that the first step in doing a definite integral is to compute the indefinite integral and that hasn’t changed. We will still compute the indefinite integral first. This means that we already know how to do these. We use the substitution rule to find the indefinite integral and then do the evaluation. There are however, two ways to deal with the evaluation step. One of the ways of doing the evaluation is the probably the most obvious at this point, but also has a point in the process where we can get in trouble if we aren’t paying attention. Let’s work an example illustrating both ways of doing the evaluation step. Example 1 Evaluate the following definite integral.

0 2 3

22 1 4t t dt

−−∫

Solution Let’s start off looking at the first way of dealing with the evaluation step. We’ll need to be careful with this method as there is a point in the process where if we aren’t paying attention we’ll get the wrong answer. Solution 1 : We’ll first need to compute the indefinite integral using the substitution rule. Note however, that we will constantly remind ourselves that this is a definite integral by putting the limits on the integral at each step. Without the limits it’s easy to forget that we had a definite integral when we’ve gotten the indefinite integral computed. In this case the substitution is,

3 2 2 11 4 1212

u t du t dt t dt du= − = − ⇒ = −

Plugging this into the integral gives,

10 02 3 22 2

032

2

12 1 46

19

t t dt u du

u

− −

−

− = −

= −

∫ ∫

Notice that we didn’t do the evaluation yet. This is where the potential problem arises with this solution method. The limits given here are from the original integral and hence are values of t. We have u’s in our solution. We can’t plug values of t in for u. Therefore, we will have to go back to t’s before we do the substitution. This is the standard step


Math 140


in the substitution process, but it is often forgotten when doing definite integrals. Note as well that in this case, if we don’t go back to t’s we will have a small problem in that one of the evaluations will end up giving us a complex number. So, finishing this problem gives,

( )

( )

( )

030 2 3 3 22

2

32

12 1 4 1 49

1 1 339 9

1 33 33 19

t t dt t−

−

− = − −

= − − −

= −

∫

So, that was the first solution method. Let’s take a look at the second method. Solution 2 : Note that this solution method isn’t really all that different from the first method. In this method we are going to remember that when doing a substitution we want to eliminate all the t’s in the integral and write everything in terms of u. When we say all here we really mean all. In other words, remember that the limits on the integral are also values of t and we’re going to convert the limits into u values. Converting the limits is pretty simple since our substitution will tell us how to relate t and u so all we need to do is plug in the original t limits into the substitution and we’ll get the new u limits. Here is the substitution (it’s the same as the first method) as well as the limit conversions.

( )( )

3 2 2

3

3

11 4 1212

2 1 4 2 33

0 1 4 0 1

u t du t dt t dt du

t u

t u

= − = − ⇒ = −

= − ⇒ = − − =

= ⇒ = − =


10 12 3 22 33

132

33

12 1 46

19

t t dt u du

u

−− = −

= −

∫ ∫

As with the first method let’s pause here a moment to remind us what we’re doing. In this case, we’ve converted the limits to u’s and we’ve also got our integral in terms of u’s and so here we can just plug the limits directly into our integral. Note that in this case we won’t plug our substitution back in. Doing this here would cause problems as we would have t’s in the integral


Math 140


and our limits would be u’s. Here’s the rest of this problem.

( ) ( )

130 2 3 22

33

32

12 1 49

1 1 133 33 33 19 9 9

t t dt u−

− = −

= − − − = −

∫

We got exactly the same answer and this time didn’t have to worry about going back to t’s in our answer. So, we’ve seen two solution techniques for computing definite integrals that require the substitution rule. Both are valid solution methods and each have their uses. We will be using the second exclusively however since it makes the evaluation step a little easier. Let’s work some more examples. Example 2 Evaluate each of the following.

(a) ( ) ( )5 52

11 2w w w dw

−+ +∫ [Solution]

(b) ( )

6

32

4 51 21 2

dxxx

−

−

−++

⌠⌡

[Solution]

(c) ( )12

02cosy y dyπ+∫ e [Solution]

(d) ( )0

3

3sin 5cos2z z dz

ππ − −

⌠⌡

[Solution]

Solution Since we’ve done quite a few substitution rule integrals to this time we aren’t going to put a lot of effort into explaining the substitution part of things here.

(a) ( ) ( )5 52

11 2w w w dw

−+ +∫

The substitution and converted limits are,

( ) ( )2 12 2 2 12

1 1 5 35

u w w du w dw w dw du

w u w u

= + = + ⇒ + =

= − ⇒ = − = ⇒ =

Sometimes a limit will remain the same after the substitution. Don’t get excited when it happens and don’t expect it to happen all the time. Here is the integral,


Math 140


( ) ( )5 3552 5

1 1

356

1

11 221 153188802

12

w w w dw u du

u

− −

−

+ + =

= =

∫ ∫

Don’t get excited about large numbers for answers here. Sometime they are. That’s life.


(b) ( )

6

32

4 51 21 2

dxxx

−

−

−++

⌠⌡

Here is the substitution and converted limits for this problem,

11 2 22

2 3 6 11

u x du dx dx du

x u x u

= + = ⇒ =

= − ⇒ = − = − ⇒ = −


( )

( )

6 113

332

112

3

4 5 1 541 2 21 2

1 2 5ln21 2 1 25ln11 5ln 32 121 2 9112 5 5ln11 ln 3

1089 2 2

dx u dux ux

u u

− −−

−−

−−

−

− = −++

= − −

= − − − − −

= − +

⌠ ⌠ ⌡⌡


(c) ( )12

02cosy y dyπ+∫ e

This integral needs to be split into two integrals since the first term doesn’t require a substitution and the second does.

( ) ( )1 1 12 2 2

0 0 02cos 2cosy yy dy dy y dyπ π+ = +∫ ∫ ∫e e

Here is the substitution and converted limits for the second term.

1

10 02 2

u y du dy dy du

y u y u

π ππ

π

= = ⇒ =

= ⇒ = = ⇒ =


Math 140


Here is the integral.

( ) ( )1 12 2 2

0 0 0

1 220

01

02

12

22cos cos

2 sin

2 2sin sin 02

21

y y

y

y dy dy u du

u

π

π

ππ

π

ππ π

π

+ = +

= +

= − + −

= − +

∫ ∫ ∫e e

e

e e

e


(d) ( )0

3

3sin 5cos2z z dz

ππ − −

⌠⌡

This integral will require two substitutions. So first split up the integral so we can do a substitution on each term.

( ) ( )0 0

0

33 3

3sin 5cos 3sin 5cos2 2z zz dz dz z dzπ

π ππ π − − = − −

⌠ ⌠ ⌡ ⌡ ∫

There are the two substitutions for these integrals.

1 22 2

0 03 6

zu du dz dz du

z u z uπ π

= = ⇒ =

= ⇒ = = ⇒ =

2 03 3

v z dv dz dz dv

z v z v

ππ π

π

= − = − ⇒ = −

= ⇒ = = ⇒ =

Here is the integral for this problem.

( ) ( ) ( )

( ) ( )

00

26 3

30

26 3

3sin 5cos 6 sin 5 cos2

6cos 5sin

5 33 3 62

3 62

z z dz u du v dv

u v

π

π ππ

ππ π

π − − = +

= − +

= − + −

= −

⌠⌡ ∫ ∫



Math 140


The next set of examples is designed to make sure that we don’t forget about a very important point about definite integrals. Example 3 Evaluate each of the following.

(a) 5

25

42 8

t dtt− −

⌠⌡

[Solution]

(b) 5

23

42 8

t dtt−

⌠⌡

[Solution]

Solution

(a) 5

25

42 8

t dtt− −

⌠⌡

Be careful with this integral. The denominator is zero at 12t = ± and both of these are in the

interval of integration. Therefore, this integrand is not continuous in the interval and so the integral can’t be done. Be careful with definite integrals and be on the lookout for division by zero problems. In the previous section they were easy to spot since all the division by zero problems that we had there were at zero. Once we move into substitution problems however they will not always be so easy to spot so make sure that you first take a quick look at the integrand and see if there are any continuity problems with the integrand and if they occur in the interval of integration.


(b) 5

23

42 8

t dtt−

⌠⌡

Now, in this case the integral can be done because the two points of discontinuity, 12t = ± , are

both outside of the interval of integration. The substitution and converted limits in this case are,

2 12 8 16

163 70 5 198

u t du t dt dz dt

t u t u

= − = − ⇒ = −

= ⇒ = − = ⇒ = −


( ) ( )( )

5 198

23 70

198

70

4 4 12 8 16

1 ln41 ln 198 ln 704

t dt dut u

u

−

−

−

−

= −−

= −

= − −

⌠ ⌠ ⌡ ⌡



Math 140


Let’s work another set of examples. These are a little tougher (at least in appearance) than the previous sets. Example 4 Evaluate each of the following.

(a) ( )( )0

ln 1cos 1x x dx

π−−∫ e e [Solution]

(b) [ ]

6

2

4ln tdt

t⌠⌡

e

e

[Solution]

(c) ( ) ( )( )

9

3

12

sec 3 tan 3

2 sec 3

P PdP

P

π

π +

⌠⌡

[Solution]

(d) ( ) ( )( )2 cos cos sinx x dxπ

π−∫ [Solution]

(e)

22

2150

wdw

w⌠⌡

e [Solution]

Solution

(a) ( )( )0

ln 1cos 1x x dx

π−−∫ e e

The limits are a little unusual in this case, but that will happen sometimes so don’t get too excited about it. Here is the substitution.

( ) ( ) ( )

0

ln 1

10 1 1 1 0

ln 1 1 1 1

x xu du dxx u

x u ππ π π−

= − = −

= ⇒ = − = − =

= − ⇒ = − = − − =

e ee

e


( )( )

( )( )

0 0

0

ln 1cos 1 cos

sin

sin sin 0 0

x x dx u du

u

π

π

π

π

−− = −

= −

= − − =

∫ ∫e e


(b) [ ]

6

2

4ln tdt

t⌠⌡

e

e

Here is the substitution and converted limits for this problem.


Math 140


2 2 6 6

1ln

ln 2 ln 6

u t du dtt

t u t u

= =

= ⇒ = = = ⇒ = =e e e e

The integral is,

[ ]6

2

46 4

2

65

2

ln

157744

5

tdt u du

t

u

=

=

=

⌠⌡ ∫

e

e


(c) ( ) ( )

( )

9

3

12

sec 3 tan 3

2 sec 3

P PdP

P

π

π +

⌠⌡

Here is the substitution and converted limits and don’t get too excited about the substitution. It’s a little messy in the case, but that can happen on occasion.

( ) ( ) ( ) ( ) ( ) 12 sec 3 3sec 3 tan 3 sec 3 tan 33

2 sec 2 212 4

2 sec 49 3

u P du P P dP P P dP du

P u

P u

π π

π π

= + = ⇒ =

= ⇒ = + = + = ⇒ = + =

Here is the integral,

( ) ( )( )

( )

19 43

2 23

1242

3

2 2

2233

sec 3 tan 3 132 sec 3

12

1 4 2 22

P PdP u du

P

u

π

π

−

+

+

=+

=

= − +

⌠⌡

∫

So, not only was the substitution messy, but we also a messy answer, but again that’s life on occasion.


(d) ( ) ( )( )2 cos cos sinx x dxπ

π−∫


Math 140


This problem not as bad as it looks. Here is the substitution and converted limits.

( )

sin cos

sin 1 sin 02 2

u x du x dx

x u x uπ ππ π

= =

= ⇒ = = = − ⇒ = − =

The cosine in the very front of the integrand will get substituted away in the differential and so this integrand actually simplifies down significantly. Here is the integral.

( ) ( )( )

( )( ) ( )( )

12

0

1

0

cos cos sin cos

sin

sin 1 sin 0

sin 1

x x dx u du

u

π

π−=

=

= −

=

∫ ∫

Don’t get excited about these kinds of answers. On occasion we will end up with trig function evaluations like this.


(e)

22

2150

wdw

w⌠⌡

e

This is also a tricky substitution (at least until you see it). Here it is,

2 2

2 2 1 12

12 1 10050

u du dw dw duw w w

w u w u

= = − ⇒ = −

= ⇒ = = ⇒ =

Here is the integral.

( )

221

2 100150

1

100

1 100

12

1212

wu

u

dw duw

= −

= −

= − −

⌠⌡

∫e e

e

e e

[Return to Problems] In this last set of examples we saw some tricky substitutions and messy limits, but these are a fact of life with some substitution problems and so we need to be prepared for dealing with them when the happen. Even and Odd Functions


Math 140


This is the last topic that we need to discuss in this chapter. It is probably better suited in the previous section, but that section has already gotten fairly large so I decided to put it here. First, recall that an even function is any function which satisfies, ( ) ( )f x f x− = Typical examples of even functions are, ( ) ( ) ( )2 cosf x x f x x= = An odd function is any function which satisfies, ( ) ( )f x f x− = − The typical examples of odd functions are, ( ) ( ) ( )3 sinf x x f x x= = There are a couple of nice facts about integrating even and odd functions over the interval [-a,a]. If f(x) is an even function then,

( ) ( )0

2a a

af x dx f x dx

−=∫ ∫

Likewise, if f(x) is an odd function then,

( ) 0a

af x dx

−=∫

Note that in order to use these facts the limit of integration must be the same number, but opposite signs! Example 5 Integrate each of the following.

(a) 2 4 2

24 1x x dx

−− +∫ [Solution]

(b) ( )10 5

10sinx x dx

−+∫ [Solution]

Solution Neither of these are terribly difficult integrals, but we can use the facts on them anyway.

(a) 2 4 2

24 1x x dx

−− +∫

In this case the integrand is even and the interval is correct so,

2 24 2 4 2

2 0

25 3

0

4 1 2 4 1

4 125 3

74815

x x dx x x dx

x x x

−− + = − +

= − +

=

∫ ∫

So, using the fact cut the evaluation in half (in essence since one of the new limits was zero).


Math 140



(b) ( )10 5

10sinx x dx

−+∫

The integrand in this case is odd and the interval is in the correct form and so we don’t even need to integrate. Just use the fact.

( )10 5

10sin 0x x dx

−+ =∫

[Return to Problems] Note that the limits of integration are important here. Take the last integral as an example. A small change to the limits will not give us zero.

( ) ( ) ( )9 5

10

468559sin cos 10 cos 9 78093.094616

x x dx−

+ = − − = −∫

The moral here is to be careful and not misuse these facts.


Math 140


Applications of Integrals

Introduction In this last chapter of this course we will be taking a look at a couple of applications of integrals. There are many other applications, however many of them require integration techniques that are typically taught in Calculus II. We will therefore be focusing on applications that can be done only with knowledge taught in this course. Because this chapter is focused on the applications of integrals it is assumed in all the examples that you are capable of doing the integrals. There will not be as much detail in the integration process in the examples in this chapter as there was in the examples in the previous chapter. Here is a listing of applications covered in this chapter. Average Function Value – We can use integrals to determine the average value of a function. Area Between Two Curves – In this section we’ll take a look at determining the area between two curves. Volumes of Solids of Revolution / Method of Rings – This is the first of two sections devoted to find the volume of a solid of revolution. In this section we look at the method of rings/disks. Volumes of Solids of Revolution / Method of Cylinders – This is the second section devoted to finding the volume of a solid of revolution. Here we will look at the method of cylinders. More Volume Problems – In this section we’ll take a look at finding the volume of some solids that are either not solids of revolutions or are not easy to do as a solid of revolution.


Math 140


Average Function Value The first application of integrals that we’ll take a look at is the average value of a function. The following fact tells us how to compute this. Average Function Value The average value of a function ( )f x over the interval [a,b] is given by,

( )1 b

avg af f x dx

b a=

− ∫

To see a justification of this formula see the Proof of Various Integral Properties section of the Extras chapter. Let’s work a couple of quick examples. Example 1 Determine the average value of each of the following functions on the given interval.

(a) ( ) ( )2 5 6cosf t t t tπ= − + on 51,2

− [Solution]

(b) ( ) ( ) ( )1 cos 2sin 2 zR z z −= e on [ ],π π− [Solution] Solution

(a) ( ) ( )2 5 6cosf t t t tπ= − + on 51,2

−

There’s really not a whole lot to do in this problem other than just use the formula.

( ) ( )

( )

522

1

52

3 2

1

52

1 5 6cos1

2 1 5 6 sin7 3 212 137 61.620993

avgf t t t dt

t t t

π

ππ

π

−

−

= − +− −

= − +

= −

= −

∫

You caught the substitution needed for the third term right? So, the average value of this function of the given interval is -1.620993.


(b) ( ) ( ) ( )1 cos 2sin 2 zR z z −= e on [ ],π π−

Again, not much to do here other than use the formula. Note that the integral will need the


Math 140


following substitution.

( )1 cos 2u z= −

Here is the average value of this function,

( ) ( ) ( )

( )

1 cos 2

1 cos 2

1 sin 2

120

zavg

z

R z dzπ

π

π

π

π π

π

−

−

−

−

=− −

=

=

∫ e

e

So, in this case the average function value is zero. Do not get excited about getting zero here. It will happen on occasion. In fact, if you look at the graph of the function on this interval it’s not too hard to see that this is the correct answer.


There is also a theorem that is related to the average function value. The Mean Value Theorem for Integrals If ( )f x is a continuous function on [a,b] then there is a number c in [a,b] such that,

( ) ( ) ( )b

af x dx f c b a= −∫

Note that this is very similar to the Mean Value Theorem that we saw in the Derivatives Applications chapter. See the Proof of Various Integral Properties section of the Extras chapter for the proof. Note that one way to think of this theorem is the following. First rewrite the result as,

( ) ( )1 b

af x dx f c

b a=

− ∫


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and from this we can see that this theorem is telling us that there is a number a c b< < such that

( )avgf f c= . Or, in other words, if ( )f x is a continuous function then somewhere in [a,b] the

function will take on its average value. Let’s take a quick look at an example using this theorem. Example 2 Determine the number c that satisfies the Mean Value Theorem for Integrals for the function ( ) 2 3 2f x x x= + + on the interval [1,4] Solution First let’s notice that the function is a polynomial and so is continuous on the given interval. This means that we can use the Mean Value Theorem. So, let’s do that.

( )( )

( )

4 2 2

1

43 2 2

1

2

2

3 2 3 2 4 1

1 3 2 3 3 23 2

99 3 9 62

70 3 92

x x dx c c

x x x c c

c c

c c

+ + = + + −

+ + = + +

= + +

8= + −

∫

This is a quadratic equation that we can solve. Using the quadratic formula we get the following two solutions,

3 67 2.5932

3 67 5.5932

c

c

− += =

− −= = −

Clearly the second number is not in the interval and so that isn’t the one that we’re after. The first however is in the interval and so that’s the number we want. Note that it is possible for both numbers to be in the interval so don’t expect only one to be in the interval.


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Area Between Curves In this section we are going to look at finding the area between two curves. There are actually two cases that we are going to be looking at. In the first case we are want to determine the area between ( )y f x= and ( )y g x= on the

interval [a,b]. We are also going to assume that ( ) ( )f x g x≥ . Take a look at the following

sketch to get an idea of what we’re initially going to look at.

In the Area and Volume Formulas section of the Extras chapter we derived the following formula for the area in this case.

( ) ( )b

aA f x g x dx= −∫ (1)

The second case is almost identical to the first case. Here we are going to determine the area between ( )x f y= and ( )x g y= on the interval [c,d] with ( ) ( )f y g y≥ .


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In this case the formula is,

( ) ( )d

cA f y g y dy= −∫ (2)

Now (1) and (2) are perfectly serviceable formulas, however, it is sometimes easy to forget that these always require the first function to be the larger of the two functions. So, instead of these formulas we will instead use the following “word” formulas to make sure that we remember that the formulas area always the “larger” function minus the “smaller” function. In the first case we will use,

upper lower

,function function

b

a

A dx a x b = − ≤ ≤

⌠⌡

(3)

In the second case we will use,

right left

,function function

d

c

A dy c y d = − ≤ ≤

⌠⌡

(4)

Using these formulas will always force us to think about what is going on with each problem and to make sure that we’ve got the correct order of functions when we go to use the formula. Let’s work an example.

Example 1 Determine the area of the region enclosed by 2y x= and y x= . Solution First of all, just what do we mean by “area enclosed by”. This means that the region we’re interested in must have one of the two curves on every boundary of the region. So, here is a graph of the two functions with the enclosed region shaded.


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Note that we don’t take any part of the region to the right of the intersection point of these two graphs. In this region there is no boundary on the right side and so is not part of the enclosed area. Remember that one of the given functions must be on the each boundary of the enclosed region. Also from this graph it’s clear that the upper function will be dependent on the range of x’s that we use. Because of this you should always sketch of a graph of the region. Without a sketch it’s often easy to mistake which of the two functions is the larger. In this case most would probably say that 2y x= is the upper function and they would be right for the vast majority of the x’s. However, in this case it is the lower of the two functions. The limits of integration for this will be the intersection points of the two curves. In this case it’s pretty easy to see that they will intersect at 0x = and 1x = so these are the limits of integration. So, the integral that we’ll need to compute to find the area is,

1 2

0

1332

0

upper lowerfunction function

2 13 3

13

b

a

A dx

x x dx

x x

= −

= −

= −

=

⌠⌡

∫

Before moving on to the next example, there are a couple of important things to note. First, in almost all of these problems a graph is pretty much required. Often the bounding region, which will give the limits of integration, is difficult to determine without a graph. Also, it can often be difficult to determine which of the functions is the upper function and with is the lower function without a graph. This is especially true in cases like the last example where the answer to that question actually depended upon the range of x’s that we were using. Finally, unlike the area under a curve that we looked at in the previous chapter the area between two curves will always be positive. If we get a negative number or zero we can be sure that we’ve made a mistake somewhere and will need to go back and find it. Note as well that sometimes instead of saying region enclosed by we will say region bounded by. They mean the same thing. Let’s work some more examples.


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Example 2 Determine the area of the region bounded by 2xy x −= e , 1y x= + , 2x = , and the

y-axis. Solution In this case the last two pieces of information, 2x = and the y-axis, tell us the right and left boundaries of the region. Also, recall that the y-axis is given by the line 0x = . Here is the graph with the enclosed region shaded in.

Here, unlike the first example, the two curves don’t meet. Instead we rely on two vertical lines to bound the left and right sides of the region as we noted above Here is the integral that will give the area.

2

0

22

04

2

2


1

1 12 2

7 3.50922 2

b

a

x

x

A dx

x x dx

x x

−

−

−

= −

= + −

= + +

= + =

⌠⌡

∫ e

e

e

Example 3 Determine the area of the region bounded by 22 10y x= + and 4 16y x= + . Solution In this case the intersection points (which we’ll need eventually) are not going to be easily identified from the graph so let’s go ahead and get them now. Note that for most of these problems you’ll not be able to accurately identify the intersection points from the graph and so you’ll need to be able to determine them by hand. In this case we can get the intersection points


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by setting the two equations equal.

( ) ( )

2

2

2 10 4 162 4 6 0

2 1 3 0

x xx x

x x

+ = +

− − =

+ − =

So it looks like the two curves will intersect at 1x = − and 3x = . If we need them we can get the y values corresponding to each of these by plugging the values back into either of the equations. We’ll leave it to you to verify that the coordinates of the two intersection points on the graph are (-1,12) and (3,28). Note as well that if you aren’t good at graphing knowing the intersection points can help in at least getting the graph started. Here is a graph of the region.

With the graph we can now identify the upper and lower function and so we can now find the enclosed area.

( )3 2

1

3 2

1

33 2

1


4 16 2 10

2 4 6

2 2 63

643

b

a

A dx

x x dx

x x dx

x x x

−

−

−

= −

= + − +

= − + +

= − + +

=

⌠⌡

∫

∫

Be careful with parenthesis in these problems. One of the more common mistakes students make with these problems is to neglect parenthesis on the second term.


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Example 4 Determine the area of the region bounded by 22 10y x= + , 4 16y x= + , 2x = − and 5x = Solution So, the functions used in this problem are identical to the functions from the first problem. The difference is that we’ve extended the bounded region out from the intersection points. Since these are the same functions we used in the previous example we won’t bother finding the intersection points again. Here is a graph of this region.

Okay, we have a small problem here. Our formula requires that one function always be the upper function and the other function always be the lower function and we clearly do not have that here. However, this actually isn’t the problem that it might at first appear to be. There are three regions in which one function is always the upper function and the other is always the lower function. So, all that we need to do is find the area of each of the three regions, which we can do, and then add them all up. Here is the area.

( ) ( ) ( )1 3 52 2 2

2 1 3

1 3 52 2 2

2 1 3

1 3 53 2 3 2 3 2

2 1 3

2 10 4 16 4 16 2 10 2 10 4 16

2 4 6 2 4 6 2 4 6

2 2 22 6 2 6 2 63 3 3

14 64 643 3 3

1423

A x x dx x x dx x x dx

x x dx x x dx x x dx

x x x x x x x x x

−

− −

−

− −

−

− −

= + − + + + − + + + − +

= − − + − + + + − −

= − − + − + + + − −

= + +

=

∫ ∫ ∫

∫ ∫ ∫


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Example 5 Determine the area of the region enclosed by siny x= , cosy x= , 2

x π= , and the

y-axis. Solution First let’s get a graph of the region.

So, we have another situation where we will need to do two integrals to get the area. The intersection point will be where sin cosx x=

in the interval. We’ll leave it to you to verify that this will be 4

x π= . The area is then,

( ) ( )

( )

24

0 4

240 4

cos sin sin cos

sin cos cos sin

2 1 2 1

2 2 2 0.828427

A x x dx x x dx

x x x x

π π

π

π π

π

= − + −

= + + − −

= − + −

= − =

∫ ∫

We will need to be careful with this next example.

Example 6 Determine the area of the region enclosed by 21 32

x y= − and 1y x= − .

Solution Don’t let the first equation get you upset. We will have to deal with these kinds of equations occasionally so we’ll need to get used to dealing with them. As always, it will help if we have the intersection points for the two curves. In this case we’ll get


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the intersection points by solving the second equation for x and the setting them equal. Here is that work,

( ) ( )

2

2

2

11 32

2 2 60 2 80 4 2

y y

y yy yy y

+ = −

+ = −

= − −

= − +

So, it looks like the two curves will intersect at 2y = − and 4y = or if we need the full coordinates they will be : (-1,-2) and (5,4). Here is a sketch of the two curves.

Now, we will have a serious problem at this point if we aren’t careful. To this point we’ve been using an upper function and a lower function. To do that here notice that there are actually two portions of the region that will have different lower functions. In the range [-2,-1] the parabola is actually both the upper and the lower function. To use the formula that we’ve been using to this point we need to solve the parabola for y. This gives,

2 6y x= ± + where the “+” gives the upper portion of the parabola and the “-” gives the lower portion. Here is a sketch of the complete area with each region shaded that we’d need if we were going to use the first formula.


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The integrals for the area would then be,

( ) ( )1 5

3 1

1 5

3 1

1 5 5

3 1 1

4 16 53 322 2

10 4

2 6 2 6 2 6 1

2 2 6 2 6 1

2 2 6 2 6 1

2 1 13 3 2

18

A x x dx x x dx

x dx x x dx

x dx x dx x dx

u u x x

−

− −

−

− −

−

− − −

−

= + − − + + + − −

= + + + − +

= + + + + − +

= + + − +

=

∫ ∫

∫ ∫

∫ ∫ ∫

While these integrals aren’t terribly difficult they are more difficult than they need to be. Recall that there is another formula for determining the area. It is,

right left

,function function

d

c

A dy c y d = − ≤ ≤

⌠⌡

and in our case we do have one function that is always on the left and the other is always on the right. So, in this case this is definitely the way to go. Note that we will need to rewrite the equation of the line since it will need to be in the form ( )x f y= but that is easy enough to do.

Here is the graph for using this formula.


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The area is,

( )4

2

2

42

2

43 2

2

right leftfunction function

11 32

1 42

1 1 46 2

18

d

c

A dy

y y dy

y y dy

y y y

−

−

−

= −

= + − −

= − + +

= − + +

=

⌠⌡

⌠⌡

⌠⌡

This is the same that we got using the first formula and this was definitely easier than the first method. So, in this last example we’ve seen a case where we could use either formula to find the area. However, the second was definitely easier. Students often come into a calculus class with the idea that the only easy way to work with functions is to use them in the form ( )y f x= . However, as we’ve seen in this previous

example there are definitely times when it will be easier to work with functions in the form

( )x f y= . In fact, there are going to be occasions when this will be the only way in which a

problem can be worked so make sure that you can deal with functions in this form.


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Let’s take a look at one more example to make sure we can deal with functions in this form.

Example 7 Determine the area of the region bounded by 2 10x y= − + and ( )22x y= − . Solution First, we will need intersection points.

( )

( )( )

22

2 2

2

10 2

10 4 40 2 4 60 2 1 3

y y

y y yy yy y

− + = −

− + = − +

= − −

= + −

The intersection points are 1y = − and 3y = . Here is a sketch of the region.

This is definitely a region where the second area formula will be easier. If we used the first formula there would be three different regions that we’d have to look at. The area in this case is,

( )

3 22

1

3 2

1

33 2

1

right leftfunction function

10 2

2 4 6

2 42 63 3

d

c

A dy

y y dy

y y dy

y y y

−

−

−

= −

= − + − −

= − + +

6 = − + + =

⌠⌡

∫

∫


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Volumes of Solids of Revolution / Method of Rings In this section we will start looking at the volume of a solid of revolution. We should first define just what a solid of revolution is. To get a solid of revolution we start out with a function,

( )y f x= , on an interval [a,b].

We then rotate this curve about a given axis to get the surface of the solid of revolution. For purposes of this discussion let’s rotate the curve about the x-axis, although it could be any vertical or horizontal axis. Doing this for the curve above gives the following three dimensional region.

What we want to do over the course of the next two sections is to determine the volume of this object. In the final the Area and Volume Formulas section of the Extras chapter we derived the following formulas for the volume of this solid.


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( ) ( )b d

a cV A x dx V A y dy= =∫ ∫

where, ( )A x and ( )A y is the cross-sectional area of the solid. There are many ways to get the

cross-sectional area and we’ll see two (or three depending on how you look at it) over the next two sections. Whether we will use ( )A x or ( )A y will depend upon the method and the axis of

rotation used for each problem. One of the easier methods for getting the cross-sectional area is to cut the object perpendicular to the axis of rotation. Doing this the cross section will be either a solid disk if the object is solid (as our above example is) or a ring if we’ve hollowed out a portion of the solid (we will see this eventually). In the case that we get a solid disk the area is,

( )2radiusA π= where the radius will depend upon the function and the axis of rotation. In the case that we get a ring the area is,

2 2outer inner

radius radiusA π

= −

where again both of the radii will depend on the functions given and the axis of rotation. Note as well that in the case of a solid disk we can think of the inner radius as zero and we’ll arrive at the correct formula for a solid disk and so this is a much more general formula to use. Also, in both cases, whether the area is a function of x or a function of y will depend upon the axis of rotation as we will see. This method is often called the method of disks or the method of rings. Let’s do an example. Example 1 Determine the volume of the solid obtained by rotating the region bounded by

2 4 5y x x= − + , 1x = , 4x = , and the x-axis about the x-axis. Solution The first thing to do is get a sketch of the bounding region and the solid obtained by rotating the region about the x-axis. Here are both of these sketches.


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Okay, to get a cross section we cut the solid at any x. Below are a couple of sketches showing a typical cross section. The sketch on the right shows a cut away of the object with a typical cross section without the caps. The sketch on the left shows just the curve we’re rotating as well as it’s mirror image along the bottom of the solid.

In this case the radius is simply the distance from the x-axis to the curve and this is nothing more than the function value at that particular x as shown above. The cross-sectional area is then,

( ) ( ) ( )22 4 3 24 5 8 26 40 25A x x x x x x xπ π= − + = − + − + Next we need to determine the limits of integration. Working from left to right the first cross section will occur at 1x = and the last cross section will occur at 4x = . These are the limits of integration.


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The volume of this solid is then,

( )4 4 3 2

1

45 4 3 2

1

8 26 40 25

1 262 20 255 3

785

b

aV A x dx

x x x x dx

x x x x x

π

π

π

=

= − + − +

= − + − +

=

∫

∫

In the above example the object was a solid object, but the more interesting objects are those that are not solid so let’s take a look at one of those. Example 2 Determine the volume of the solid obtained by rotating the portion of the region

bounded by 3y x= and 4xy = that lies in the first quadrant about the y-axis.

Solution First, let’s get a graph of the bounding region and a graph of the object. Remember that we only want the portion of the bounding region that lies in the first quadrant. There is a portion of the bounding region that is in the third quadrant as well, but we don't want that for this problem.

There are a couple of things to note with this problem. First, we are only looking for the volume of the “walls” of this solid, not the complete interior as we did in the last example. Next, we will get our cross section by cutting the object perpendicular to the axis of rotation. The cross section will be a ring (remember we are only looking at the walls) for this example and it will be horizontal at some y. This means that the inner and outer radius for the ring will be x values and so we will need to rewrite our functions into the form ( )x f y= . Here are the

functions written in the correct form for this example.


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33

44

y x x yxy x y

= ⇒ =

= ⇒ =

Here are a couple of sketches of the boundaries of the walls of this object as well as a typical ring. The sketch on the left includes the back portion of the object to give a little context to the figure on the right.

The inner radius in this case is the distance from the y-axis to the inner curve while the outer radius is the distance from the y-axis to the outer curve. Both of these are then x distances and so are given by the equations of the curves as shown above. The cross-sectional area is then,

( ) ( ) ( )( ) ( )22 3 2 64 16A y y y y yπ π= − = −

Working from the bottom of the solid to the top we can see that the first cross-section will occur at 0y = and the last cross-section will occur at 2y = . These will be the limits of integration. The volume is then,

( )2 2 6

0

23 7

0

16

16 13 7

51221

d

cV A y dy

y y dy

y y

π

π

π

=

= −

= −

=

∫

∫

With these two examples out of the way we can now make a generalization about this method. If we rotate about a horizontal axis (the x-axis for example) then the cross sectional area will be a


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function of x. Likewise, if we rotate about a vertical axis (the y-axis for example) then the cross sectional area will be a function of y. The remaining two examples in this section will make sure that we don’t get too used to the idea of always rotating about the x or y-axis. Example 3 Determine the volume of the solid obtained by rotating the region bounded by

2 2y x x= − and y x= about the line 4y = . Solution First let’s get the bounding region and the solid graphed.

Again, we are going to be looking for the volume of the walls of this object. Also since we are rotating about a horizontal axis we know that the cross-sectional area will be a function of x. Here are a couple of sketches of the boundaries of the walls of this object as well as a typical ring. The sketch on the left includes the back portion of the object to give a little context to the figure on the right.


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Now, we’re going to have to be careful here in determining the inner and outer radius as they aren’t going to be quite as simple they were in the previous two examples. Let’s start with the inner radius as this one is a little clearer. First, the inner radius is NOT x. The distance from the x-axis to the inner edge of the ring is x, but we want the radius and that is the distance from the axis of rotation to the inner edge of the ring. So, we know that the distance from the axis of rotation to the x-axis is 4 and the distance from the x-axis to the inner ring is x. The inner radius must then be the difference between these two. Or, inner radius 4 x= − The outer radius works the same way. The outer radius is,

( )2 2outer radius 4 2 2 4x x x x= − − = − + + Note that given the location of the typical ring in the sketch above the formula for the outer radius may not look quite right but it is in fact correct. As sketched the outer edge of the ring is below the x-axis and at this point the value of the function will be negative and so when we do the subtraction in the formula for the outer radius we’ll actually be subtracting off a negative number which has the net effect of adding this distance onto 4 and that gives the correct outer radius. Likewise, if the outer edge is above the x-axis, the function value will be positive and so we’ll be doing an honest subtraction here and again we’ll get the correct radius in this case. The cross-sectional area for this case is,

( ) ( ) ( )( ) ( )2 22 4 3 22 4 4 4 5 24A x x x x x x x xπ π= − + + − − = − − +


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The first ring will occur at 0x = and the last ring will occur at 3x = and so these are our limits of integration. The volume is then,

( )3 4 3 2

0

35 4 3 2

0

4 5 24

1 5 125 3

1535

b

aV A x dx

x x x x dx

x x x x

π

π

π

=

= − − +

= − − +

=

∫

∫

Example 4 Determine the volume of the solid obtained by rotating the region bounded by

2 1y x= − and 1y x= − about the line 1x = − . Solution As with the previous examples, let’s first graph the bounded region and the solid.

Now, let’s notice that since we are rotating about a vertical axis and so the cross-sectional area will be a function of y. This also means that we are going to have to rewrite the functions to also get them in terms of y.

2

2 1 14

1 1

yy x x

y x x y

= − ⇒ = +

= − ⇒ = +

Here are a couple of sketches of the boundaries of the walls of this object as well as a typical ring. The sketch on the left includes the back portion of the object to give a little context to the figure on the right.


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The inner and outer radius for this case is both similar and different from the previous example. This example is similar in the sense that the radii are not just the functions. In this example the functions are the distances from the y-axis to the edges of the rings. The center of the ring however is a distance of 1 from the y-axis. This means that the distance from the center to the edges is a distance from the axis of rotation to the y-axis (a distance of 1) and then from the y-axis to the edge of the rings. So, the radii are then the functions plus 1 and that is what makes this example different from the previous example. Here we had to add the distance to the function value whereas in the previous example we needed to subtract the function from this distance. Note that without sketches the radii on these problems can be difficult to get. So, in summary, we’ve got the following for the inner and outer radius for this example.

2 2

outer radius 1 1 2

inner radius 1 1 24 4

y yy y

= + + = +

= + + = +

The cross-sectional area it then,

( ) ( )22 4

22 2 44 16y yA y y yπ π

= + − + = −

The first ring will occur at 0y = and the final ring will occur at 4y = and so these will be our limits of integration. The volume is,


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( )4 4

0

42 5

0

416

1280

965

d

cV A y dy

yy dy

y y

π

π

π

=

= −

= −

=

⌠⌡

∫


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Volumes of Solids of Revolution / Method of Cylinders In the previous section we started looking at finding volumes of solids of revolution. In that section we took cross sections that were rings or disks, found the cross-sectional area and then used the following formulas to find the volume of the solid.

( ) ( )b d


In the previous section we only used cross sections that were in the shape of a disk or a ring. This however does not always need to be the case. We can use any shape for the cross sections as long as it can be expanded or contracted to completely cover the solid we’re looking at. This is a good thing because as our first example will show us we can’t always use rings/disks. Example 1 Determine the volume of the solid obtained by rotating the region bounded by

( )( )21 3y x x= − − and the x-axis about the y-axis. Solution As we did in the previous section, let’s first graph the bounded region and solid. Note that the bounded region here is the shaded portion shown. The curve is extended out a little past this for the purposes of illustrating what the curve looks like.

So, we’ve basically got something that’s roughly doughnut shaped. If we were to use rings on this solid here is what a typical ring would look like.

This leads to several problems. First, both the inner and outer radius are defined by the same function. This, in itself, can be dealt with on occasion as we saw in a example in the Area


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Between Curves section. However, this usually means more work than other methods so it’s often not the best approach. This leads to the second problem we got here. In order to use rings we would need to put this function into the form ( )x f y= . That is NOT easy in general for a cubic polynomial and in

other cases may not even be possible to do. Even when it is possible to do this the resulting equation is often significantly messier than the original which can also cause problems. The last problem with rings in this case is not so much a problem as its just added work. If we were to use rings the limit would be y limits and this means that we will need to know how high the graph goes. To this point the limits of integration have always been intersection points that were fairly easy to find. However, in this case the highest point is not an intersection point, but instead a relative maximum. We spent several sections in the Applications of Derivatives chapter talking about how to find maximum values of functions. However, finding them can, on occasion, take some work. So, we’ve seen three problems with rings in this case that will either increase our work load or outright prevent us from using rings. What we need to do is to find a different way to cut the solid that will give us a cross-sectional area that we can work with. One way to do this is to think of our solid as a lump of cookie dough and instead of cutting it perpendicular to the axis of rotation we could instead center a cylindrical cookie cutter on the axis of rotation and push this down into the solid. Doing this would give the following picture,

Doing this gives us a cylinder or shell in the object and we can easily find its surface area. The surface area of this cylinder is,

( ) ( )( )( ) ( )( )( )( )

2

4 3 2

2 radius height

2 1 3

2 7 15 9

A x

x x x

x x x x

π

π

π

=

= − −

= − + −

Notice as well that as we increase the radius of the cylinder we will completely cover the solid and so we can use this in our formula to find the volume of this solid. All we need are limits of integration. The first cylinder will cut into the solid at 1x = and as we increase x to 3x = we


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will completely cover both sides of the solid since expanding the cylinder in one direction will automatically expand it in the other direction as well. The volume of this solid is then,

( )3 4 3 2

1

35 4 3 2

1

2 7 15 9

1 7 92 55 4 2

245

b

aV A x dx

x x x x dx

x x x x

π

π

π

=

= − + −

= − + −

=

∫

∫

The method used in the last example is called the method of cylinders or method of shells. The formula for the area in all cases will be, ( ) ( )2 radius heightA π= There are a couple of important differences between this method and the method of rings/disks that we should note before moving on. First, rotation about a vertical axis will give an area that is a function of x and rotation about a horizontal axis will give an area that is a function of y. This is exactly opposite of the method of rings/disks. Second, we don’t take the complete range of x or y for the limits of integration as we did in the previous section. Instead we take a range of x or y that will cover one side of the solid. As we noted in the first example if we expand out the radius to cover one side we will automatically expand in the other direction as well to cover the other side. Let’s take a look at another example. Example 2 Determine the volume of the solid obtained by rotating the region bounded by

3y x= , 8x = and the x-axis about the x-axis. Solution First let’s get a graph of the bounded region and the solid.


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Okay, we are rotating about a horizontal axis. This means that the area will be a function of y and so our equation will also need to be wrote in ( )x f y= form.

33y x x y= ⇒ = As we did in the ring/disk section let’s take a couple of looks at a typical cylinder. The sketch on the left shows a typical cylinder with the back half of the object also in the sketch to give the right sketch some context. The sketch on the right contains a typical cylinder and only the curves that define the edge of the solid.

In this case the width of the cylinder is not the function value as it was in the previous example. In this case the function value is the distance between the edge of the cylinder and the y-axis. The distance from the edge out to the line is 8x = and so the width is then 38 y− . The cross sectional area in this case is,

( ) ( ) ( )( )( )( )

3

4

2 radius width

2 8

2 8

A y

y y

y y

π

π

π

=

= −

= −

The first cylinder will cut into the solid at 0y = and the final cylinder will cut in at 2y = and so these are our limits of integration. The volume of this solid is,


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( )2 4

0

22 5

0

2 8

12 45

965

d

cV A y dy

y y dy

y y

π

π

π

=

= −

= −

=

∫

∫

The remaining examples in this section will have axis of rotation about axis other than the x and y-axis. As with the method of rings/disks we will need to be a little careful with these. Example 3 Determine the volume of the solid obtained by rotating the region bounded by

2 1y x= − and 1y x= − about the line 6x = . Solution Here’s a graph of the bounded region and solid.

Here are our sketches of a typical cylinder. Again, the sketch on the left is here to provide some context for the sketch on the right.


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Okay, there is a lot going on in the sketch to the left. First notice that the radius is not just an x or y as it was in the previous two cases. In this case x is the distance from the x-axis to the edge of the cylinder and we need the distance from the axis of rotation to the edge of the cylinder. That means that the radius of this cylinder is 6 x− . Secondly, the height of the cylinder is the difference of the two functions in this case. The cross sectional area is then,

( ) ( )( )( ) ( )( )2

2 radius height

2 6 2 1 1

2 7 6 12 1 2 1

A x

x x x

x x x x x

π

π

π

=

= − − − +

= − + + − − −

Now the first cylinder will cut into the solid at 1x = and the final cylinder will cut into the solid at 5x = so there are our limits. Here is the volume.

( )

( ) ( ) ( )

5 2

1

53 3 53 2 2 2 2

1

2 7 6 12 1 2 1

1 7 4 42 6 8 1 1 13 2 3 5

136215

27215

b

aV A x dx

x x x x x dx

x x x x x x

π

π

π

π

=

= − + + − − −

= − + + − − − − −

=

=

∫

∫


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The integration of the last term is a little tricky so let’s do that here. It will use the substitution, 1 1u x du dx x u= − = = +

( )

( ) ( )

12

3 12 2

5 32 2

5 32 2

2 1 2 1

2

2 225 3

4 41 15 3

x x dx u u du

u u du

u u c

x x c

− = +

= +

= + +

= − + − +

⌠⌡∫

∫

We saw one of these kinds of substitutions back in the substitution section. Example 4 Determine the volume of the solid obtained by rotating the region bounded by

( )22x y= − and y x= about the line 1y = − . Solution We should first get the intersection points there.

( )

( )( )

2

2

2

2

4 40 5 40 4 1

y y

y y yy yy y

= −

= − +

= − +

= − −

So, the two curves will intersect at 1y = and 4y = . Here is a sketch of the bounded region and the solid.

Here are our sketches of a typical cylinder. The sketch on the left is here to provide some context for the sketch on the right.


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Here’s the cross sectional area for this cylinder.

( ) ( ) ( )( ) ( )( )( )

2

3 2

2 radius width

2 1 2

2 4 4

A y

y y y

y y y

π

π

π

=

= + − −

= − + + −

The first cylinder will cut into the solid at 1y = and the final cylinder will cut in at 4y = . The volume is then,

( )4 3 2

1

44 3 2

1

2 4 4

1 4 12 44 3 2

632

d

cV A y dy

y y y dy

y y y y

π

π

π

=

= − + + −

= − + + −

=

∫

∫


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More Volume Problems In this section we’re going to take a look at some more volume problems. However, the problems we’ll be looking at here will not be solids of revolution as we looked at in the previous two sections. There are many solids out there that cannot be generated as solids of revolution, or at least not easily and so we need to take a look at how to do some of these problems. Now, having said that these will not be solids of revolutions they will still be worked in pretty much the same manner. For each solid we’ll need to determine the cross-sectional area, either

( )A x or ( )A y , and they use the formulas we used in the previous two sections,

( ) ( )b d


The “hard” part of these problems will be determining what the cross-sectional area for each solid is. Each problem will be different and so each cross-sectional area will be determined by different means. Also, before we proceed with any examples we need to acknowledge that the integrals in this section might look a little tricky at first. There are going to be very few numbers in these problems. All of the examples in this section are going to be more general derivation of volume formulas for certain solids. As such we’ll be working with things like circles of radius r and we’ll not be giving a specific value of r and we’ll have heights of h instead of specific heights, etc. All the letters in the integrals are going to make the integrals look a little tricky, but all you have to remember is that the r’s and the h’s are just letters being used to represent a fixed quantity for the problem, i.e. it is a constant. So when we integrate we only need to worry about the letter in the differential as that is the variable we’re actually integrating with respect to. All other letters in the integral should be thought of as constants. If you have trouble doing that, just think about what you’d do if the r was a 2 or the h was a 3 for example. Let’s start with a simple example that we don’t actually need to do an integral that will illustrate how these problems work in general and will get us used to seeing multiple letters in integrals. Example 1 Find the volume of a cylinder of radius r and height h. Solution Now, as we mentioned before starting this example we really don’t need to use an integral to find this volume, but it is a good example to illustrate the method we’ll need to use for these types of problems. We’ll start off with the sketch of the cylinder below.


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We’ll center the cylinder on the x-axis and the cylinder will start at 0x = and end at x h= as shown. Note that we’re only choosing this particular set up to get an integral in terms of x and to make the limits nice to deal with. There are many other orientations that we could use. What we need here is to get a formula for the cross-sectional area at any x. In this case the cross-sectional area is constant and will be a disk of radius r. Therefore, for any x we’ll have the following cross-sectional area,

( ) 2A x rπ=

Next the limits for the integral will be 0 x h≤ ≤ since that is the range of x in which the cylinder lives. Here is the integral for the volume,

2 2 2 2

00 0

h h hV r dx r dx r x r hπ π π π= = = =∫ ∫

So, we get the expected formula. Also, recall we are using r to represent the radius of the cylinder. While r can clearly take different values it will never change once we start the problem. Cylinders do not change their radius in the middle of a problem and so as we move along the center of the cylinder (i.e. the x-axis) r is a fixed number and won’t change. In other words it is a constant that will not change as we change the x. Therefore, because we integrated with respect to x the r will be a constant as far as the integral is concerned. The r can then be pulled out of the integral as shown (although that’s not required, we just did it to make the point). At this point we’re just integrating dx and we know how to do that. When we evaluate the integral remember that the limits are x values and so we plug into the x and NOT the r. Again, remember that r is just a letter that is being used represent the radius of the cylinder and, once we start the integration, is assumed to be a fixed constant. As noted before we started this example if you’re having trouble with the r just think of what you’d do if there was a 2 there instead of an r. In this problem, because we’re integrating with respect to x, both the 2 and the r will behave in the same manner. Note however that you should NEVER actually replace the r with a 2 as that WILL lead to a wrong answer. You should just think of what you would do IF the r was a 2.


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So, to work these problems we’ll first need to get a sketch of the solid with a set of x and y axes to help us see what’s going on. At the very least we’ll need the sketch to get the limits of the integral, but we will often need it to see just what the cross-sectional area is. Once we have the sketch we’ll need to determine a formula for the cross-sectional area and then do the integral. Let’s work a couple of more complicated examples. In these examples the main issue is going to be determining what the cross-sectional areas are. Example 2 Find the volume of a pyramid whose base is a square with sides of length L and whose height is h. Solution Let’s start off with a sketch of the pyramid. In this case we’ll center the pyramid on the y-axis and to make the equations easier we are going to position the point of the pyramid at the origin.

Now, as shown here the cross-sectional area will be a function of y and it will also be a square with sides of length s. The area of the square is easy, but we’ll need to get the length of the side in terms of y. To determine this consider the figure on the right above. If we look at the pyramid directly from the front we’ll see that we have two similar triangles and we know that the ratio of any two sides of similar triangles must be equal. In other words, we know that,

s y y Ls L yL h h h

= ⇒ = =

So, the cross-sectional area is then,

( )2

2 22

LA y s yh

= =


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The limit for the integral will be 0 y h≤ ≤ and the volume will be,

2 2 2

2 2 3 22 2 20

0 0

1 13 3

hhhL L LV y dy y dy y L h

h h h = = = =

⌠⌡ ∫

Again, do not get excited about the L and the h in the integral. Once we start the problem if we change y they will not change and so they are constants as far as the integral is concerned and can get pulled out of the integral. Also, remember that when we evaluate will only plug the limits into the variable we integrated with respect to, y in this case. Before we proceed with some more complicated examples we should once again remind you to not get excited about the other letters in the integrals. If we’re integrating with respect to x or y then all other letters in the formula that represent fixed quantities (i.e. radius, height, length of a side, etc.) are just constants and can be treated as such when doing the integral. Now let’s do some more examples. Example 3 For a sphere of radius r find the volume of the cap of height h. Solution A sketch is probably best to illustrate what we’re after here.

We are after the top portion of the sphere and the height of this is portion is h. In this case we’ll use a cross-sectional area that starts at the bottom of the cap, which is at y r h= − , and moves up towards the top, which is at y r= . So, each cross-section will be a disk of radius x. It is a little easier to see that the radius will be x if we look at it from the top as shown in the sketch to the right above. The area of this disk is then,

2xπ This is a problem however as we need the cross-sectional area in terms of y. So, what we really


Math 140


need to determine what 2x will be for any given y at the cross-section. To get this let’s look at the sphere from the front.

In particular look at the triangle POR. Because the point R lies on the sphere itself we can see that the length of the hypotenuse of this triangle (the line OR) is r, the radius of the sphere. The line PR has a length of x and the line OP has length y so by the Pythagorean Theorem we know, 2 2 2 2 2 2x y r x r y+ = ⇒ = − So, we now know what 2x will be for any given y and so the cross-sectional area is,

( ) ( )2 2A y r yπ= −

As we noted earlier the limits on y will be r h y r− ≤ ≤ and so the volume is,

( ) ( ) ( ) ( )2 2 2 3 2 3 21 1 13 3 3

rr

r h r hV r y dy r y y h r h h r hπ π π π

− −= − = − = − = −∫

In the previous example we again saw an r in the integral. However, unlike the previous two examples it was not multiplied times the x or the y and so could not be pulled out of the integral. In this case it was like we were integrating 24 y− and we know how integrate that. So, in this

case we need to treat the 2r like the 4 and so when we integrate that we’ll pick up a y.


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Example 4 Find the volume of a wedge cut out of a cylinder of radius r if the angle between the top and bottom of the wedge is 6

π . Solution We should really start off with a sketch of just what we’re looking for here.

On the left we see how the wedge is being cut out of the cylinder. The base of the cylinder is the circle give by 2 2 2x y r+ = and the angle between this circle and the top of the wedge is 6

π . The sketch in the upper right position is the actual wedge itself. Given the orientation of the axes here we get the portion of the circle with positive y and so we can write the equation of the circle as

2 2y r x= − since we only need the positive y values. Note as well that this is the reason for the way we oriented the axes here. We get positive y’s and we can write the equation of the circle as a function only of x’s. Now, as we can see in the two sketches of the wedge the cross-sectional area will be a right triangle and the area will be a function of x as we move from the back of the cylinder, at x r= − , to the front of the cylinder, at x r= . The right angle of the triangle will be on the circle itself while the point on the x-axis will have an interior angle of 6

π . The base of the triangle will have a length of y and using a little right triangle trig we see that the height of the rectangle is, ( ) 1

6 3height tany yπ= =

So, we now know the base and height of our triangle, in terms of y, and we have an equation for y in terms of x and so we can see that the area of the triangle, i.e. the cross-sectional area is,

( ) ( )( ) ( ) ( )2 2 2 2 2 21 1 1 1 12 23 3 2 3

A x y y r x r x r x= = − − = −

The limits on x are r x r− ≤ ≤ and so the volume is then,

( ) ( ) 322 2 2 31 1 132 3 2 3 3 3

rr rr r

V r x dx r x x− −

= − = − =∫


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The next example is very similar to the previous one except it can be a little difficult to visualize the solid itself. Example 5 Find the volume of the solid whose base is a disk of radius r and whose cross-sections are equilateral triangles. Solution Let’s start off with a couple of sketches of this solid. The sketch on the left is from the “front” of the solid and the sketch on the right is more from the top of the solid.

The base of this solid is the disk of radius r and we move from the back of the disk at x r= − to the front of the disk at x r= we form equilateral triangles to form the solid. A sample equilateral triangle, which is also the cross-sectional area, is shown above to hopefully make it a little clearer how the solid is formed. Now, let’s get a formula for the cross-sectional area. Let’s start with the two sketches below.

In the left hand sketch we are looking at the solid from directly above and notice that we “reoriented” the sketch a little to put the x and y-axis in the “normal” orientation. The solid


Math 140


vertical line in this sketch is the cross-sectional area. From this we can see that the cross-section occurs at a given x and the top half will have a length of y where the value of y will be the y-coordinate of the point on the circle and so is,

2 2y r x= − Also, because the cross-section is an equilateral triangle that is centered on the x-axis the bottom half will also have a length of y. Thus the base of the cross-section must have a length of 2y. The sketch to the right is of one of the cross-sections. As noted above the base of the triangle has a length of 2y. Also note that because it is an equilateral triangle the angles are all 3

π . If we divide the cross-section in two (as shown with the dashed line) we now have two right triangles and using right triangle trig we can see that the length of the dashed line is,

( )3dashed line tan 3y yπ= =

Therefore the height of the cross-section is 3 y . Because the cross-section is a triangle we know that that it’s area must then be,

( ) ( ) ( ) ( )( ) ( )2 2 2 2 2 21 12 22 3 2 3 3A x y y r x r x r x= = − − = −

Note that we used the cross-sectional area in terms of x because each of the cross-sections is perpendicular to the x-axis and this pretty much forces us to integrate with respect to x. The volume of the solid is then,

( ) ( ) ( )2 2 2 3 31 43 3

3 3rr

r rV x r x dx r x x r

− −= − = − =∫

The final example we’re going to work here is a little tricky both in seeing how to set it up and in doing the integral.


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Example 6 Find the volume of a torus with radii r and R. Solution First, just what is a torus? A torus is a donut shaped solid that is generated by rotating the circle of radius r and centered at (R, 0) about the y-axis. This is shown in the sketch to the left below.

One of the trickiest parts of this problem is seeing what the cross-sectional area needs to be. There is an obvious one. Most people would probably think of using the circle of radius r that we’re rotating about the y-axis as the cross-section. It is definitely one of the more obvious choices, however setting up an integral using this is not so easy. So, what we’ll do is use a cross-section as shown in the sketch to the right above. If we cut the torus perpendicular to the y-axis we’ll get a cross-section of a ring and finding the area of that shouldn’t be too bad. To do that let’s take a look at the two sketches below.

The sketch to the left is a sketch of the full cross-section. The sketch to the left is more important however. This is a sketch of the circle that we are rotating about the y-axis. Included is a line representing where the cross-sectional area would be in the torus. Notice that the inner radius will always be the left portion of the circle and the outer radius will always be the right portion of the circle. Now, we know that the equation of this is,

( )2 2 2x R y r− + =


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and so if we solve for x we can get the equations for the left and right sides as shown above in the sketch. This however means that we also now have equations for the inner and outer radii.

2 2 2 2inner radius : outer radius :x R r y x R r y= − − = + − The cross-sectional area is then,

( ) ( ) ( )

( ) ( )( )

2 2

2 22 2 2 2

2 2 2 2 2 2 2 2 2 2

2 2

outer radius inner radius

2 2

4

A y

R r y R r y

R R r y r y R R r y r y

R r y

π π

π

π

π

= −

= + − − − − = + − + − − − − + −

= −

Next, the lowest cross-section will occur at y r= − and the highest cross-section will occur at y r= and so the limits for the integral will be r y r− ≤ ≤ .

The integral giving the volume is then,

2 2 2 2 2 2

0 04 2 4 8

r r r

rV R r y dy R r y dy R r y dyπ π π

−= − = − = −∫ ∫ ∫

Note that we used the fact that because the integrand is an even function and we’re integrating over [ ],r r− we could change the lower limit to zero and double the value of the integral. We

saw this fact back in the Substitution Rule for Definite Integrals section. We’ve now reached the second really tricky part of this example. With the knowledge that we’ve currently got at this point this integral is not possible to do. It requires something called a trig substitution and that is a topic for Calculus II. Luckily enough for us, and this is the tricky part, in this case we can actually determine what the integrals value is using what we know about integrals. Just for a second let’s think about a different problem. Let’s suppose we wanted to use an integral to determine the area under the portion of the circle of radius r and centered at the origin that is in the first quadrant. There are a couple of ways to do this, but to match what we’re doing here let’s do the following. We know that the equation of the circle is 2 2 2x y r+ = and if we solve for x the equation of the circle in the first (and fourth for that matter) quadrant is,

2 2x r y= − If we want an integral for the area in the first quadrant we can think of this area as the region


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between the curve 2 2x r y= − and the y-axis for 0 y r≤ ≤ and this is,

2 2

0

rA r y dy= −∫

In other words, this integral represents one quarter of the area of a circle of radius r and from basic geometric formulas we now know that this integral must have the value,

2 2 2140

rA r y dy rπ= − =∫

So, putting all this together the volume of the torus is then,

( )2 2 2 2 2140

8 8 2r

V R r y dy R r R rπ π π π= − = =∫


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Extras

Introduction In this chapter material that didn’t fit into other sections for a variety of reasons. Also, in order to not obscure the mechanics of actually working problems, most of the proofs of various facts and formulas are in this chapter as opposed to being in the section with the fact/formula. This chapter contains those topics. Proof of Various Integral Facts/Formulas/Properties – Here we will give the proofs of some of the facts and formulas from the Integral Chapter as well as a couple from the Applications of Integrals chapter. Area and Volume Formulas – Here is the derivation of the formulas for finding area between two curves and finding the volume of a solid of revolution. Types of Infinity – This is a discussion on the types of infinity and how these affect certain limits. Summation Notation – Here is a quick review of summation notation. Constant of Integration – This is a discussion on a couple of subtleties involving constants of integration that many students don’t think about.


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Proof of Various Integral Facts/Formulas/Properties In this section we’ve got the proof of several of the properties we saw in the Integrals Chapter as well as a couple from the Applications of Integrals Chapter.

Proof of : ( ) ( )k f x dx k f x dx=∫ ∫ where k is any number.

This is a very simple proof. Suppose that ( )F x is an anti-derivative of ( )f x , i.e.

( ) ( )F x f x′ = . Then by the basic properties of derivatives we also have that,

( )( ) ( ) ( )k F x k F x k f x′ ′= =

and so ( )k F x is an anti-derivative of ( )k f x , i.e. ( )( ) ( )k F x k f x′ = . In other words,

( ) ( ) ( )k f x dx k F x c k f x dx= + =∫ ∫

Proof of : ( ) ( ) ( ) ( )f x g x dx f x dx g x dx± = ±∫ ∫ ∫

This is also a very simple proof Suppose that ( )F x is an anti-derivative of ( )f x and that

( )G x is an anti-derivative of ( )g x . So we have that ( ) ( )F x f x′ = and ( ) ( )G x g x′ = .

Basic properties of derivatives also tell us that

( ) ( )( ) ( ) ( ) ( ) ( )F x G x F x G x f x g x′ ′ ′± = ± = ±

and so ( ) ( )F x G x+ is an anti-derivative of ( ) ( )f x g x+ and ( ) ( )F x G x− is an anti-

derivative of ( ) ( )f x g x− . In other words,

( ) ( ) ( ) ( ) ( ) ( )f x g x dx F x G x c f x dx g x dx± = ± + = ±∫ ∫ ∫


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Proof of : ( ) ( )b a

a bf x dx f x dx= −∫ ∫

From the definition of the definite integral we have,

( ) ( )*

1lim

nb

ia n i

b af x dx f x x xn→∞

=

−= ∆ ∆ =∑∫

and we also have,

( ) ( )*

1lim

na

ib n i

a bf x dx f x x xn→∞

=

−= ∆ ∆ =∑∫

Therefore,

( ) ( )

( ) ( )

( )

( ) ( )

*

1

*

1

*

1

*

1

lim

lim

lim

lim

nb

ia n i

n

in i

n

in i

n a

i bn i

b af x dx f xna b

f xn

a bf xn

a bf x f x dxn

→∞=

→∞=

→∞=

→∞=

−=

− −=

− = −

−= − = −

∑∫

∑

∑

∑ ∫

Proof of : ( ) 0a

af x dx =∫


( ) ( )

( ) ( )

*

1

*

1

lim 0

lim 0

lim 0

0

na

ia n in

in i

n

a af x dx f x x xn

f x

→∞=

→∞=

→∞

−= ∆ ∆ = =

=

=

=

∑∫

∑


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Proof of : ( ) ( )b b

a acf x dx c f x dx=∫ ∫


( ) ( )

( )

( )

( )

*

1

*

1

*

1

lim

lim

lim

nb

ia n in

in in

in ib

a

c f x dx c f x x

c f x x

c f x x

c f x dx

→∞=

→∞=

→∞=

= ∆

= ∆

= ∆

=

∑∫

∑

∑

∫

Remember that we can pull constants out of summations and out of limits.

Proof of : ( ) ( ) ( ) ( )b b b

a a af x g x dx f x dx g x dx± = ±∫ ∫ ∫

First we’ll prove the formula for “+”. From the definition of the definite integral we have,

( ) ( ) ( ) ( )( )

( ) ( )

( ) ( )

( ) ( )

* *

1

* *

1 1

* *

1 1

lim

lim

lim lim

nb

i ia n i

n n

i in i i

n n

i in ni ib b

a a

f x g x dx f x g x x

f x x g x x

f x x g x x

f x dx g x dx

→∞=

→∞= =

→∞ →∞= =

+ = + ∆

= ∆ + ∆

= ∆ + ∆

= +

∑∫

∑ ∑

∑ ∑

∫ ∫

To prove the formula for “-” we can either redo the above work with a minus sign instead of a plus sign or we can use the fact that we now know this is true with a plus and using the properties proved above as follows.

( ) ( ) ( ) ( )( )

( ) ( )( )

( ) ( )

b b

a a

b b

a a

b b

a a

f x g x dx f x g x dx

f x dx g x dx

f x dx g x dx

− = + −

= + −

= −

∫ ∫

∫ ∫

∫ ∫


Math 140


Proof of : ( )b

ac dx c b a= −∫ , c is any number.

If we define ( )f x c= then from the definition of the definite integral we have,

( )

( )

( )

( )( )

*

1

1

lim

lim

lim

lim

b b

a a

n

in i

n

n i

n

n

c dx f x dx

b af x x xn

b acn

b acnn

c b a

c b a

→∞=

→∞=

→∞

→∞

=

−= ∆ ∆ =

− =

−

=

= −

= −

∫ ∫

∑

∑

Proof of : If ( ) 0f x ≥ for a x b≤ ≤ then ( ) 0b

af x dx ≥∫ .


( ) ( )*

1lim

nb

ia n i

b af x dx f x x xn→∞

=

−= ∆ ∆ =∑∫

Now, by assumption ( ) 0f x ≥ and we also have 0x∆ > and so we know that

( )*

10

n

ii

f x x=

∆ ≥∑

So, from the basic properties of limits we then have,

( )*

1lim lim 0 0

n

in nif x x

→∞ →∞=

∆ ≥ =∑

But the left side is exactly the definition of the integral and so we have,

( ) ( )*

1lim 0

nb

ia n if x dx f x x

→∞=

= ∆ ≥∑∫


Math 140


Proof of : If ( ) ( )f x g x≥ for a x b≤ ≤ then ( ) ( )b b

a af x dx g x dx≥∫ ∫ .

Since we have ( ) ( )f x g x≥ then we know that ( ) ( ) 0f x g x− ≥ on a x b≤ ≤ and so by

Property 8 proved above we know that,

( ) ( ) 0b

af x g x dx− ≥∫

We also know from Property 4 that,

( ) ( ) ( ) ( )b b b

a a af x g x dx f x dx g x dx− = −∫ ∫ ∫

So, we then have,

( ) ( )

( ) ( )

0b b

a a

b b

a a

f x dx g x dx

f x dx g x dx

− ≥

≥

∫ ∫

∫ ∫

Proof of : If ( )m f x M≤ ≤ for a x b≤ ≤ then ( ) ( ) ( )b

am b a f x dx M b a− ≤ ≤ −∫ .

Give ( )m f x M≤ ≤ we can use Property 9 on each inequality to write,

( )b b b

a a am dx f x dx M dx≤ ≤∫ ∫ ∫

Then by Property 7 on the left and right integral to get,

( ) ( ) ( )b

am b a f x dx M b a− ≤ ≤ −∫

Proof of : ( ) ( )b b


First let’s note that we can say the following about the function and the absolute value,

( ) ( ) ( )f x f x f x− ≤ ≤


Math 140


If we now use Property 9 on each inequality we get,

( ) ( ) ( )b b b

a a af x dx f x dx f x dx− ≤ ≤∫ ∫ ∫

We know that we can factor the minus sign out of the left integral to get,

( ) ( ) ( )b b b

a a af x dx f x dx f x dx− ≤ ≤∫ ∫ ∫

Finally, recall that if p b≤ then b p b− ≤ ≤ and of course this works in reverse as well so we

then must have,

( ) ( )b b


Fundamental Theorem of Calculus, Part I If ( )f x is continuous on [a,b] then,

( ) ( )x

ag x f t dt= ∫

is continuous on [a,b] and it is differentiable on ( ),a b and that,

( ) ( )g x f x′ = Proof Suppose that x and x h+ are in ( ),a b . We then have,

( ) ( ) ( ) ( )x h x

a ag x h g x f t dt f t dt

++ − = −∫ ∫

Now, using Property 5 of the Integral Properties we can rewrite the first integral and then do a little simplification as follows.

( ) ( ) ( ) ( )( ) ( )

( )

x x h x

a x a

x h

x

g x h g x f t dt f t dt f t dt

f t dt

+

+

+ − = + −

=

∫ ∫ ∫

∫

Finally assume that 0h ≠ and we get,

( ) ( ) ( )1 x h

x

g x h g xf t dt

h h++ −

= ∫ (1)


Math 140


Let’s now assume that 0h > and since we are still assuming that x h+ are in ( ),a b we know

that ( )f x is continuous on [ ],x x h+ and so by the Extreme Value Theorem we know that

there are numbers c and d in [ ],x x h+ so that ( )f c m= is the absolute minimum of ( )f x in

[ ],x x h+ and that ( )f d M= is the absolute maximum of ( )f x in [ ],x x h+ .

So, by Property 10 of the Integral Properties we then know that we have,

( )x h

xmh f t dt Mh

+≤ ≤∫

Or,

( ) ( ) ( )x h

xf c h f t dt f d h

+≤ ≤∫

Now divide both sides of this by h to get,

( ) ( ) ( )1 x h

xf c f t dt f d

h+

≤ ≤∫

and then use (1) to get,

( ) ( ) ( ) ( )g x h g xf c f d

h+ −

≤ ≤ (2)

Next, if 0h < we can go through the same argument above except we’ll be working on

[ ],x h x+ to arrive at exactly the same inequality above. In other words, (2) is true provided

0h ≠ . Now, if we take 0h → we also have c x→ and d x→ because both c and d are between x and x h+ . This means that we have the following two limits. ( ) ( ) ( ) ( ) ( ) ( )

0 0lim lim lim limh c x h d x

f c f c f x f d f d f x→ → → →

= = = =

The Squeeze Theorem then tells us that,

( ) ( ) ( )

0limh

g x h g xf x

h→

+ −= (3)

but the left side of this is exactly the definition of the derivative of ( )g x and so we get that,

( ) ( )g x f x′ =


Math 140


So, we’ve shown that ( )g x is differentiable on ( ),a b .

Now, the Theorem at the end of the Definition of the Derivative section tells us that ( )g x is also

continuous on ( ),a b . Finally, if we take x a= or x b= we can go through a similar argument

we used to get (3) using one-sided limits to get the same result and so the theorem at the end of the Definition of the Derivative section will also tell us that ( )g x is continuous at x a= or

x b= and so in fact ( )g x is also continuous on [ ],a b .

Fundamental Theorem of Calculus, Part II Suppose ( )f x is a continuous function on [a,b] and also suppose that ( )F x is any anti-

derivative for ( )f x . Then,

( ) ( ) ( ) ( )b b

aaf x dx F x F b F a= = −∫

Proof

First let ( ) ( )x

ag x f t dt= ∫ and then we know from Part I of the Fundamental Theorem of

Calculus that ( ) ( )g x f x′ = and so ( )g x is an anti-derivative of ( )f x on [a,b]. Further

suppose that ( )F x is any anti-derivative of ( )f x on [a,b] that we want to choose. So, this

means that we must have,

( ) ( )g x F x′ ′=

Then, by Fact 2 in the Mean Value Theorem section we know that ( )g x and ( )F x can differ

by no more than an additive constant on ( ),a b . In other words for a x b< < we have,

( ) ( )F x g x c= +

Now because ( )g x and ( )F x are continuous on [a,b], if we take the limit of this as x a+→

and x b−→ we can see that this also holds if x a= and x b= . So, for a x b≤ ≤ we know that ( ) ( )F x g x c= + . Let’s use this and the definition of ( )g x to

do the following.


Math 140


( ) ( ) ( )( ) ( )( )( ) ( )

( ) ( )

( )

( )

0

b a

a a

b

a

b

a

F b F a g b c g a c

g b g a

f t dt f t dt

f t dt

f x dx

− = + − +

= −

= +

= +

=

∫ ∫

∫

∫

Note that in the last step we used the fact that the variable used in the integral does not matter and so we could change the t’s to x’s.

Average Function Value The average value of a function ( )f x over the interval [a,b] is given by,

( )1 b

avg af f x dx

b a=

− ∫

Proof We know that the average value of n numbers is simply the sum of all the numbers divided by n so let’s start off with that. Let’s take the interval [a,b] and divide it into n subintervals each of length,

b axn−

∆ =

Now from each of these intervals choose the points * * *

1 2, , , nx x x… and note that it doesn’t really matter how we choose each of these numbers as long as they come from the appropriate interval.

We can then compute the average of the function values ( ) ( ) ( )* * *1 2, , , nf x f x f x… by

computing,

( ) ( ) ( )* * *

1 2 nf x f x f xn

+ + +L (4)

Now, from our definition of x∆ we can get the following formula for n.

b an

x−

=∆


Math 140


and we can plug this into (4) to get,

( ) ( ) ( ) ( ) ( ) ( )

( ) ( ) ( )

( )

* * ** * *1 21 2

* * *1 2

*

1

1

1

nn

n

n

ii

f x f x f x xf x f x f xb a b a

x

f x x f x x f x xb a

f x xb a =

+ + + ∆+ + + =− −∆

= ∆ + ∆ + + ∆ −

= ∆− ∑

LL

L

Let’s now increase n. Doing this will mean that we’re taking the average of more and more function values in the interval and so the larger we chose n the better this will approximate the average value of the function. If we then take the limit as n goes to infinity we should get the average function value. Or,

( ) ( )* *

1 1

1 1lim limn n

avg i in ni if f x x f x x

b a b a→∞ →∞= =

= ∆ = ∆− −∑ ∑

We can factor the 1

b a− out of the limit as we’ve done and now the limit of the sum should look familiar as that is the definition of the definite integral. So, putting in definite integral we get the formula that we were after.

( )1 b

avg af f x dx

b a=

− ∫

The Mean Value Theorem for Integrals If ( )f x is a continuous function on [a,b] then there is a number c in [a,b] such that,

( ) ( ) ( )b


Proof Let’s start off by defining,

( ) ( )x

aF x f t dt= ∫

Since ( )f x is continuous we know from the Fundamental Theorem of Calculus, Part I that

( )F x is continuous on [a,b], differentiable on (a,b) and that ( ) ( )F x f x′ = .


Math 140


Now, from the Mean Value Theorem we know that there is a number c such that a c b< < and that, ( ) ( ) ( ) ( )F b F a F c b a′− = −

However we know that ( ) ( )F c f c′ = and,

( ) ( ) ( ) ( ) ( ) 0b b a

a a aF b f t dt f x dx F a f t dt= = = =∫ ∫ ∫

So, we then have,

( ) ( ) ( )b


Work The work done by the force ( )F x (assuming that ( )F x is continuous) over the range

a x b≤ ≤ is,

( )b

aW F x dx= ∫

Proof Let’s start off by dividing the range a x b≤ ≤ into n subintervals of width x∆ and from each of these intervals choose the points * * *

1 2, , , nx x x… . Now, if n is large and because ( )F x is continuous we can assume that ( )F x won’t vary by

much over each interval and so in the ith interval we can assume that the force is approximately

constant with a value of ( ) ( )*iF x F x≈ . The work on each interval is then approximately,

( )*i iW F x x≈ ∆

The total work over a x b≤ ≤ is then approximately,

( )*

1 0

n n

i ii i

W W F x x= =

≈ = ∆∑ ∑

Finally, if we take the limit of this as n goes to infinity we’ll get the exact work done. So,

( )*

0lim

n

in iW F x x

→∞=

= ∆∑


Math 140


This is, however, nothing more than the definition of the definite integral and so the work done by the force ( )F x over a x b≤ ≤ is,

( )b

aW F x dx= ∫


Math 140


Area and Volume Formulas In this section we will derive the formulas used to get the area between two curves and the volume of a solid of revolution. Area Between Two Curves We will start with the formula for determining the area between ( )y f x= and ( )y g x= on the

interval [a,b]. We will also assume that ( ) ( )f x g x≥ on [a,b].

We will now proceed much as we did when we looked that the Area Problem in the Integrals Chapter. We will first divide up the interval into n equal subintervals each with length,

b axn−

∆ =

Next, pick a point in each subinterval, *ix , and we can then use rectangles on each interval as

follows.

The height of each of these rectangles is given by,

( ) ( )* *i if x g x−

and the area of each rectangle is then,

( ) ( )( )* *i if x g x x− ∆

So, the area between the two curves is then approximated by,

( ) ( )( )* *

1

n

i ii

A f x g x x=

≈ − ∆∑

The exact area is,

( ) ( )( )* *

1lim

n

i in iA f x g x x

→∞=

= − ∆∑


Math 140


Now, recalling the definition of the definite integral this is nothing more than,

( ) ( )b

aA f x g x dx= −∫

The formula above will work provided the two functions are in the form ( )y f x= and

( )y g x= . However, not all functions are in that form. Sometimes we will be forced to work

with functions in the form between ( )x f y= and ( )x g y= on the interval [c,d] (an interval of

y values…). When this happens the derivation is identical. First we will start by assuming that ( ) ( )f y g y≥

on [c,d]. We can then divide up the interval into equal subintervals and build rectangles on each of these intervals. Here is a sketch of this situation.

Following the work from above, we will arrive at the following for the area,

( ) ( )d

cA f y g y dy= −∫

So, regardless of the form that the functions are in we use basically the same formula.


Math 140


Volumes for Solid of Revolution Before deriving the formula for this we should probably first define just what a solid of revolution is. To get a solid of revolution we start out with a function, ( )y f x= , on an interval [a,b].

We then rotate this curve about a given axis to get the surface of the solid of revolution. For purposes of this derivation let’s rotate the curve about the x-axis. Doing this gives the following three dimensional region.

We want to determine the volume of the interior of this object. To do this we will proceed much as we did for the area between two curves case. We will first divide up the interval into n subintervals of width,

b axn−

∆ =

We will then choose a point from each subinterval, *ix .

Now, in the area between two curves case we approximated the area using rectangles on each subinterval. For volumes we will use disks on each subinterval to approximate the area. The area

of the face of each disk is given by ( )*iA x and the volume of each disk is

( )*i iV A x x= ∆

Here is a sketch of this,


Math 140


The volume of the region can then be approximated by,

( )*

1

n

ii

V A x x=

≈ ∆∑

The exact volume is then,

( )

( )

*

1lim

n

in ib

a

V A x x

A x dx

→∞=

= ∆

=

∑

∫

So, in this case the volume will be the integral of the cross-sectional area at any x, ( )A x . Note

as well that, in this case, the cross-sectional area is a circle and we could go farther and get a formula for that as well. However, the formula above is more general and will work for any way of getting a cross section so we will leave it like it is. In the sections where we actually use this formula we will also see that there are ways of generating the cross section that will actually give a cross-sectional area that is a function of y instead of x. In these cases the formula will be,

( ) ,d

cV A y dy c y d= ≤ ≤∫

In this case we looked at rotating a curve about the x-axis, however, we could have just as easily rotated the curve about the y-axis. In fact we could rotate the curve about any vertical or horizontal axis and in all of these, case we can use one or both of the following formulas.

( ) ( )b d



Math 140


Types of Infinity Most students have run across infinity at some point in time prior to a calculus class. However, when they have dealt with it, it was just a symbol used to represent a really, really large positive or really, really large negative number and that was the extent of it. Once they get into a calculus class students are asked to do some basic algebra with infinity and this is where they get into trouble. Infinity is NOT a number and for the most part doesn’t behave like a number. However, despite that we’ll think of infinity in this section as a really, really, really large number that is so large there isn’t another number larger than it. This is not correct of course, but may help with the discussion in this section. Note as well that everything that we’ll be discussing in this section applies only to real numbers. If you move into complex numbers for instance things can and do change. So, let’s start thinking about addition with infinity. When you add two non-zero numbers you get a new number. For example, 4 7 11+ = . With infinity this is not true. With infinity you have the following.

where a a∞ + = ∞ ≠ −∞∞ + ∞ = ∞

In other words, a really, really large positive number ( ∞ ) plus any positive number, regardless of the size, is still a really, really large positive number. Likewise, you can add a negative number (i.e. 0a < ) to a really, really large positive number and stay really, really large and positive. So, addition involving infinity can be dealt with in an intuitive way if you’re careful. Note as well that the a must NOT be negative infinity. If it is, there are some serious issues that we need to deal with as we’ll see in a bit. Subtraction with negative infinity can also be dealt with in an intuitive way in most cases as well. A really, really large negative number minus any positive number, regardless of its size, is still a really, really large negative number. Subtracting a negative number (i.e. 0a < ) from a really, really large negative number will still be a really, really large negative number. Or,

where a a−∞ − = −∞ ≠ −∞−∞ − ∞ = −∞

Again, a must not be negative infinity to avoid some potentially serious difficulties. Multiplication can be dealt with fairly intuitively as well. A really, really large number (positive, or negative) times any number, regardless of size, is still a really, really large number we’ll just need to be careful with signs. In the case of multiplication we have

( ) ( ) ( )( )

( ) ( ) ( ) ( ) ( ) ( )if 0 if 0a a a a∞ = ∞ > ∞ = −∞ <

∞ ∞ = ∞ −∞ −∞ = ∞ −∞ ∞ = −∞

What you know about products of positive and negative numbers is still true here. Some forms of division can be dealt with intuitively as well. A really, really large number divided by a number that isn’t too large is still a really, really large number.


Math 140


if 0, if 0,

if 0, if 0,

a a a aa a

a a a aa a

∞ ∞= ∞ > ≠ ∞ = −∞ < ≠ −∞

−∞ −∞= −∞ > ≠ ∞ = ∞ < ≠ −∞

Division of a number by infinity is somewhat intuitive, but there are a couple of subtleties that you need to be aware of. When we talk about division by infinity we are really talking about a limiting process in which the denominator is going towards infinity. So, a number that isn’t too large divided an increasingly large number is an increasingly small number. In other words in the limit we have,

0 0a a= =

∞ −∞

So, we’ve dealt with almost every basic algebraic operation involving infinity. There are two cases that that we haven’t dealt with yet. These are

? ?± ∞∞ − ∞ = =

± ∞

The problem with these two cases is that intuition doesn’t really help here. A really, really large number minus a really, really large number can be anything ( −∞ , a constant, or ∞ ). Likewise, a really, really large number divided by a really, really large number can also be anything ( ±∞ – this depends on sign issues, 0, or a non-zero constant). What we’ve got to remember here is that there are really, really large numbers and then there are really, really, really large numbers. In other words, some infinities are larger than other infinities. With addition, multiplication and the first sets of division we worked this wasn’t an issue. The general size of the infinity just doesn’t affect the answer in those cases. However, with the subtraction and division cases listed above, it does matter as we will see. Here is one way to think of this idea that some infinities are larger than others. This is a fairly dry and technical way to think of this and your calculus problems will probably never use this stuff, but this it is a nice way of looking at this. Also, please note that I’m not trying to give a precise proof of anything here. I’m just trying to give you a little insight into the problems with infinity and how some infinities can be thought of as larger than others. For a much better (and definitely more precise) discussion see, http://www.math.vanderbilt.edu/~schectex/courses/infinity.pdf Let’s start by looking at how many integers there are. Clearly, I hope, there are an infinite number of them, but let’s try to get a better grasp on the “size” of this infinity. So, pick any two integers completely at random. Start at the smaller of the two and list, in increasing order, all the integers that come after that. Eventually we will reach the larger of the two integers that you picked.


http://www.math.vanderbilt.edu/~schectex/courses/infinity.pdf

Math 140


Depending on the relative size of the two integers it might take a very, very long time to list all the integers between them and there isn’t really a purpose to doing it. But, it could be done if we wanted to and that’s the important part. Because we could list all these integers between two randomly chosen integers we say that the integers are countably infinite. Again, there is no real reason to actually do this, it is simply something that can be done if we should chose to do so. In general a set of numbers is called countably infinite if we can find a way to list them all out. In a more precise mathematical setting this is generally done with a special kind of function called a bijection that associates each number in the set with exactly one of the positive integers. To see some more details of this see the pdf given above. It can also be shown that the set of all fractions are also countably infinite, although this is a little harder to show and is not really the purpose of this discussion. To see a proof of this see the pdf given above. It has a very nice proof of this fact. Let’s contrast this by trying to figure out how many numbers there are in the interval (0,1). By numbers, I mean all possible fractions that lie between zero and one as well as all possible decimals (that aren’t fractions) that lie between zero and one. The following is similar to the proof given in the pdf above, but was nice enough and easy enough (I hope) that I wanted to include it here. To start let’s assume that all the numbers in the interval (0,1) are countably infinite. This means that there should be a way to list all of them out. We could have something like the following,

1

2

3

4

0.6920960.1710340.9936710.045908

xxxx

====

LLLL

M M

Now, select the ith decimal out of ix as shown below

1

2

3

4

0.6920960.1710340.9936710.045908

xxxx

====

LLLL

M M

and form a new number with these digits. So, for our example we would have the number 0.6739x = L In this new decimal replace all the 3’s with a 1 and replace every other numbers with a 3. In the case of our example this would yield the new number


Math 140


0.3313x = L Notice that this number is in the interval (0,1) and also notice that given how we choose the digits of the number this number will not be equal to the first number in our list, 1x , because the first digit of each is guaranteed to not be the same. Likewise, this new number will not get the same number as the second in our list, 2x , because the second digit of each is guaranteed to not be the

same. Continuing in this manner we can see that this new number we constructed, x , is guaranteed to not be in our listing. But this contradicts the initial assumption that we could list out all the numbers in the interval (0,1). Hence, it must not be possible to list out all the numbers in the interval (0,1). Sets of numbers, such as all the numbers in (0,1), that we can’t write down in a list are called uncountably infinite. The reason for going over this is the following. An infinity that is uncountably infinite is significantly larger than an infinity that is only countably infinite. So, if we take the difference of two infinities we have a couple of possibilities.

( ) ( )( ) ( )

uncountable countable

countable uncountable

∞ − ∞ = ∞

∞ − ∞ = −∞

Notice that we didn’t put down a difference of two infinities of the same type. Depending upon the context there might still have some ambiguity about just what the answer would be in this case, but that is a whole different topic. We could also do something similar for quotients of infinities.

( )( )( )( )

countable

uncountable

uncountable

countable

0∞

=∞

∞= ∞

∞

Again, we avoided a quotient of two infinities of the same type since, again depending upon the context, there might still be ambiguities about its value. So, that’s it and hopefully you’ve learned something from this discussion. Infinity simply isn’t a number and because there are different kinds of infinity it generally doesn’t behave as a number does. Be careful when dealing with infinity.


Math 140


Summation Notation In this section we need to do a brief review of summation notation or sigma notation. We’ll start out with two integers, n and m, with n m< and a list of numbers denoted as follows, 1 2 2 1, , , , , ,n n n m m ma a a a a a+ + − −… We want to add them up, in other words we want, 1 2 2 1n n n m m ma a a a a a+ + − −+ + + + + +… For large lists this can be a fairly cumbersome notation so we introduce summation notation to denote these kinds of sums. The case above is denoted as follows.

1 2 2 1

m

i n n n m m mi n

a a a a a a a+ + − −=

= + + + + + +∑ …

The i is called the index of summation. This notation tells us to add all the ai’s up for all integers starting at n and ending at m. For instance,

( ) ( ) ( ) ( ) ( )

4

06

2 1 4 9 5 11 6 13 9 11 13

44

* * * * *1 2 3 4

1

0 1 2 3 4 163 2.716661 0 1 1 1 2 1 3 1 4 1 60

2 2 2 2 16 32 64

i

i i

i

ii

ii

x x x x x x x

f x f x f x f x f x

=

+

=

=

= + + + + = =+ + + + + +

= + + = + +

= + + +

∑

∑

∑

Properties Here are a couple of formulas for summation notation.

1. 0 0

n n

i ii i i i

ca c a= =

=∑ ∑ where c is any number. So, we can factor constants out of a summation.

2. ( )0 0 0

n n n

i i i ii i i i i i

a b a b= = =

± = ±∑ ∑ ∑ So we can break up a summation across a sum or difference.

Note that we started the series at 0i to denote the fact that they can start at any value of i that we

need them to. Also note that while we can break up sums and differences as we did in 2 above we can’t do the same thing for products and quotients. In other words,

( ) 0

0 0 0 0

0

n

in n n ni ii

i i i i ni i i i i i i i i

ii i

aaa b a bb b

=

= = = =

=

≠ ≠

∑∑ ∑ ∑ ∑

∑


Math 140


Formulas Here are a couple of nice formulas that we will find useful in a couple of sections. Note that these formulas are only true if starting at 1i = . You can, of course, derive other formulas from these for different starting points if you need to.

1. 1

n

ic cn

=

=∑

2. ( )1

12

n

i

n ni

=

+=∑

3. ( ) ( )2

1

1 2 16

n

i

n n ni

=

+ +=∑

4. ( ) 23

1

12

n

i

n ni

=

+ =

∑

Here is a quick example on how to use these properties to quickly evaluate a sum that would not be easy to do by hand. Example 1 Using the formulas and properties from above determine the value of the following summation.

( )100

2

13 2

ii

=

−∑

Solution The first thing that we need to do is square out the stuff being summed and then break up the summation using the properties as follows,

( )100 100

2 2

1 1100 100 100

2

1 1 1100 100 100

2

1 1 1

3 2 9 12 4

9 12 4

9 12 4

i i

i i i

i i i

i i i

i i

i i

= =

= = =

= = =

− = − +

= − +

= − +

∑ ∑

∑ ∑ ∑

∑ ∑ ∑

Now, using the formulas, this is easy to compute,

( ) ( ) ( ) ( ) ( )1002

1

100 101 100 101 2013 2 9 100 12 4

2 61293700

ii

=

− = − +

=

∑

Doing this by hand would definitely taken some time and there’s a good chance that we might have made a minor mistake somewhere along the line.


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Constants of Integration In this section we need to address a couple of topics about the constant of integration. Throughout most calculus classes we play pretty fast and loose with it and because of that many students don’t really understand it or how it can be important. First, let’s address how we play fast and loose with it. Recall that technically when we integrate a sum or difference we are actually doing multiple integrals. For instance,

4 2 4 215 9 15 9x x dx x dx x dx− −− = −∫ ∫ ∫ Upon evaluating each of these integrals we should get a constant of integration for each integral since we really are doing two integrals.

4 2 4 2

5 1

5 1

15 9 15 9

3 93 9

x x dx x dx x dx

x c x kx x c k

− −

−

−

− = −

= + + +

= + + +

∫ ∫ ∫

Since there is no reason to think that the constants of integration will be the same from each integral we use different constants for each integral. Now, both c and k are unknown constants and so the sum of two unknown constants is just an unknown constant and we acknowledge that by simply writing the sum as a c. So, the integral is then,

4 2 5 115 9 3 9x x dx x x c− −− = + +∫ We also tend to play fast and loose with constants of integration in some substitution rule problems. Consider the following problem,

( ) ( ) 1cos 1 2 sin 1 2 cos sin 1 22

x x dx u u du u x+ + + = + = +∫ ∫

Technically when we integrate we should get,

( ) ( ) ( )1cos 1 2 sin 1 2 sin cos2

x x dx u u c+ + + = − +∫

Since the whole integral is multiplied by 1

2 , the whole answer, including the constant of

integration, should be multiplied by 12 . Upon multiplying the 1

2 through the answer we get,

( ) ( ) 1 1cos 1 2 sin 1 2 sin cos2 2 2

cx x dx u u+ + + = − +∫


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However, since the constant of integration is an unknown constant dividing it by 2 isn’t going to change that fact so we tend to just write the fraction as a c.

( ) ( ) 1 1cos 1 2 sin 1 2 sin cos2 2

x x dx u u c+ + + = − +∫

In general, we don’t really need to worry about how we’ve played fast and loose with the constant of integration in either of the two examples above. The real problem however is that because we play fast and loose with these constants of integration most students don’t really have a good grasp oF them and don’t understand that there are times where the constants of integration are important and that we need to be careful with them. To see how a lack of understanding about the constant of integration can cause problems consider the following integral.

12

dxx

⌠⌡

This is a really simple integral. However, there are two ways (both simple) to integrate it and that is where the problem arises. The first integration method is to just break up the fraction and do the integral.

1 1 1 1 ln2 2 2

dx dx x cx x

= = +⌠ ⌠ ⌡ ⌡

The second way is to use the following substitution.

12 22

u x du dx dx du= = ⇒ =

1 1 1 1 1ln ln 22 2 2 2

dx du u c x cx u

= = + = +⌠ ⌠ ⌡ ⌡

Can you see the problem? We integrated the same function and got very different answers. This doesn’t make any sense. Integrating the same function should give us the same answer. We only used different methods to do the integral and both are perfectly legitimate integration methods. So, how can using different methods produce different answer? The first thing that we should notice is that because we used a different method for each there is no reason to think that the constant of integration will in fact be the same number and so we really should use different letters for each. More appropriate answers would be,

1 1 1 1ln ln 22 2 2 2

dx x c dx x kx x

= + = +⌠ ⌠ ⌡ ⌡


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Now, let’s take another look at the second answer. Using a property of logarithms we can write the answer to the second integral as follows,

( )

1 1 ln 22 2

1 ln 2 ln21 1ln ln 22 2

dx x kx

x k

x k

= +

= + +

= + +

⌠⌡

Upon doing this we can see that the answers really aren’t that different after all. In fact they only differ by a constant and we can even find a relationship between c and k. It looks like,

1 ln 22

c k= +

So, without a proper understanding of the constant of integration, in particular using different integration techniques on the same integral will likely produce a different constant of integration, we might never figure out why we got “different” answers for the integral. Note as well that getting answers that differ by a constant doesn’t violate any principles of calculus. In fact, we’ve actually seen a fact that suggested that this might happen. We saw a fact in the Mean Value Theorem section that said that if ( ) ( )f x g x′ ′= then ( ) ( )f x g x c= + . In

other words, if two functions have the same derivative then they can differ by no more than a constant. This is exactly what we’ve got here. The two functions,

( ) ( )1 1ln ln 22 2

f x x g x x= =

have exactly the same derivative,

12x

and as we’ve shown they really only differ by a constant. There is another integral that also exhibits this behavior. Consider,

( ) ( )sin cosx x dx∫ There are actually three different methods for doing this integral. Method 1 : This method uses a trig formula, ( ) ( ) ( )sin 2 2sin cosx x x= Using this formula (and a quick substitution) the integral becomes,


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( ) ( ) ( ) ( ) 11 1sin cos sin 2 cos 22 4

x x dx x dx x c= = − +∫ ∫

Method 2 : This method uses the substitution, ( ) ( )cos sinu x du x dx= = −

( ) ( ) ( )2 22 2

1 1sin cos cos2 2

x x dx u du u c x c= − = − + = − +∫ ∫

Method 3 : Here is another substitution that could be done here as well.

( ) ( )sin cosu x du x dx= =

( ) ( ) ( )2 23 3

1 1sin cos sin2 2

x x dx u du u c x c= = + = +∫ ∫

So, we’ve got three different answers each with a different constant of integration. However, according to the fact above these three answers should only differ by a constant since they all have the same derivative. In fact they do only differ by a constant. We’ll need the following trig formulas to prove this. ( ) ( ) ( ) ( ) ( )2 2 2 2cos 2 cos sin cos sin 1x x x x x= − + = Start with the answer from the first method and use the double angle formula above.

( ) ( )( )2 21

1 cos sin4

x x c− − +

Now, from the second identity above we have, ( ) ( )2 2sin 1 cosx x= − so, plug this in,

( ) ( )( )( ) ( )( )

( )

2 2 21 1

21

1 1cos 1 cos 2cos 14 4

1 1cos2 4

x x c x c

x c

− − − + = − − +

= − + +

This is then answer we got from the second method with a slightly different constant. In other words,

2 114

c c= +

We can do a similar manipulation to get the answer from the third method. Again, starting with the answer from the first method use the double angle formula and then substitute in for the cosine instead of the sine using,


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( ) ( )2 2cos 1 sinx x= − Doing this gives,

( )( ) ( )( ) ( )( )

( )

2 2 21 1

21

1 11 sin sin 1 2sin4 4

1 1sin2 4

x x c x c

x c

− − − + = − − +

= − +

which is the answer from the third method with a different constant and again we can relate the two constants by,

3 114

c c= − +

So, what have we learned here? Hopefully we’ve seen that constants of integration are important and we can’t forget about them. We often don’t work with them in a Calculus I course, yet without a good understanding of them we would be hard pressed to understand how different integration methods and apparently produce different answers.


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Integration Techniques

Introduction In this chapter we are going to be looking at various integration techniques. There are a fair number of them and some will be easier than others. The point of the chapter is to teach you these new techniques and so this chapter assumes that you’ve got a fairly good working knowledge of basic integration as well as substitutions with integrals. In fact, most integrals involving “simple” substitutions will not have any of the substitution work shown. It is going to be assumed that you can verify the substitution portion of the integration yourself. Also, most of the integrals done in this chapter will be indefinite integrals. It is also assumed that once you can do the indefinite integrals you can also do the definite integrals and so to conserve space we concentrate mostly on indefinite integrals. There is one exception to this and that is the Trig Substitution section and in this case there are some subtleties involved with definite integrals that we’re going to have to watch out for. Outside of that however, most sections will have at most one definite integral example and some sections will not have any definite integral examples. Here is a list of topics that are covered in this chapter. Integration by Parts – Of all the integration techniques covered in this chapter this is probably the one that students are most likely to run into down the road in other classes. Integrals Involving Trig Functions – In this section we look at integrating certain products and quotients of trig functions. Trig Substitutions – Here we will look using substitutions involving trig functions and how they can be used to simplify certain integrals. Partial Fractions – We will use partial fractions to allow us to do integrals involving some rational functions. Integrals Involving Roots – We will take a look at a substitution that can, on occasion, be used with integrals involving roots. Integrals Involving Quadratics – In this section we are going to look at some integrals that involve quadratics.


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Using Integral Tables – Here we look at using Integral Tables as well as relating new integrals back to integrals that we already know how to do. Integration Strategy – We give a general set of guidelines for determining how to evaluate an integral. Improper Integrals – We will look at integrals with infinite intervals of integration and integrals with discontinuous integrands in this section. Comparison Test for Improper Integrals – Here we will use the Comparison Test to determine if improper integrals converge or diverge. Approximating Definite Integrals – There are many ways to approximate the value of a definite integral. We will look at three of them in this section.


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Integration by Parts Let’s start off with this section with a couple of integrals that we should already be able to do to get us started. First let’s take a look at the following.

x xdx c= +∫e e So, that was simple enough. Now, let’s take a look at,

2xx dx∫ e

To do this integral we’ll use the following substitution.

2 122

u x du x dx x dx du= = ⇒ =

2 21 1 1

2 2 2u ux xx dx du c c= = + = +∫ ∫e e e e

Again, simple enough to do provided you remember how to do substitutions. By the way make sure that you can do these kinds of substitutions quickly and easily. From this point on we are going to be doing these kinds of substitutions in our head. If you have to stop and write these out with every problem you will find that it will take you significantly longer to do these problems. Now, let’s look at the integral that we really want to do.

6xx dx∫ e If we just had an x by itself or 6xe by itself we could do the integral easily enough. But, we don’t have them by themselves, they are instead multiplied together. There is no substitution that we can use on this integral that will allow us to do the integral. So, at this point we don’t have the knowledge to do this integral. To do this integral we will need to use integration by parts so let’s derive the integration by parts formula. We’ll start with the product rule.

( )f g f g f g′ ′ ′= + Now, integrate both sides of this.

( )f g dx f g f g dx′ ′ ′= +∫ ∫ The left side is easy enough to integrate and we’ll split up the right side of the integral.

fg f g dx f g dx′ ′= +∫ ∫


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Note that technically we should have had a constant of integration show up on the left side after doing the integration. We can drop it at this point since other constants of integration will be showing up down the road and they would just end up absorbing this one. Finally, rewrite the formula as follows and we arrive at the integration by parts formula.

f g dx fg f g dx′ ′= −∫ ∫

This is not the easiest formula to use however. So, let’s do a couple of substitutions.

( ) ( )

( ) ( )u f x v g x

du f x dx dv g x dx

= =

′ ′= =

Both of these are just the standard Calc I substitutions that hopefully you are used to by now. Don’t get excited by the fact that we are using two substitutions here. They will work the same way. Using these substitutions gives us the formula that most people think of as the integration by parts formula.

u dv uv v du= −∫ ∫

To use this formula we will need to identify u and dv, compute du and v and then use the formula. Note as well that computing v is very easy. All we need to do is integrate dv.

v dv= ∫ So, let’s take a look at the integral above that we mentioned we wanted to do. Example 1 Evaluate the following integral. 6xx dx∫ e Solution So, on some level, the problem here is the x that is in front of the exponential. If that wasn’t there we could do the integral. Notice as well that in doing integration by parts anything that we choose for u will be differentiated. So, it seems that choosing u x= will be a good choice since upon differentiating the x will drop out. Now that we’ve chosen u we know that dv will be everything else that remains. So, here are the choices for u and dv as well as du and v.

6

6 616

x

x x

u x dv dx

du dx v dx

= =

= = =∫

e

e e


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6 6 6

6 6

16 6

16 36

x x x

x x

xx dx dx

x c

= −

= − +

⌠⌡∫ e e e

e e

Once we have done the last integral in the problem we will add in the constant of integration to get our final answer. Next, let’s take a look at integration by parts for definite integrals. The integration by parts formula for definite integrals is, Integration by Parts, Definite Integrals

b bb

aa au dv uv v du= −∫ ∫

Note that the b

auv in the first term is just the standard integral evaluation notation that you should

be familiar with at this point. All we do is evaluate the term, uv in this case, at b then subtract off the evaluation of the term at a. At some level we don’t really need a formula here because we know that when doing definite integrals all we need to do is do the indefinite integral and then do the evaluation. Let’s take a quick look at a definite integral using integration by parts. Example 2 Evaluate the following integral.

2 6

1

xx dx−∫ e

Solution This is the same integral that we looked at in the first example so we’ll use the same u and dv to get,

22 26 6 6

1 11

2 26 6

1 1

12 6

16 6

16 3611 736 36

x x x

x x

xx dx dx

x

− −−

− −

−

= −

= −

= +

∫ ∫e e e

e e

e e

Since we need to be able to do the indefinite integral in order to do the definite integral and doing the definite integral amounts to nothing more than evaluating the indefinite integral at a couple of points we will concentrate on doing indefinite integrals in the rest of this section. In fact,


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throughout most of this chapter this will be the case. We will be doing far more indefinite integrals than definite integrals. Let’s take a look at some more examples. Example 3 Evaluate the following integral.

( )3 5 cos4tt dt +

⌠⌡

Solution There are two ways to proceed with this example. For many, the first thing that they try is multiplying the cosine through the parenthesis, splitting up the integral and then doing integration by parts on the first integral. While that is a perfectly acceptable way of doing the problem it’s more work than we really need to do. Instead of splitting the integral up let’s instead use the following choices for u and dv.

3 5 cos

4

3 4sin4

tu t dv dt

tdu dt v

= + = = =


( ) ( )

( )

3 5 cos 4 3 5 sin 12 sin4 4 4

4 3 5 sin 48cos4 4

t t tt dt t dt

t tt c

+ = + −

= + + +

⌠ ⌠ ⌡ ⌡

Notice that we pulled any constants out of the integral when we used the integration by parts formula. We will usually do this in order to simplify the integral a little. Example 4 Evaluate the following integral. ( )2 sin 10w w dw∫ Solution For this example we’ll use the following choices for u and dv.

( )

( )

2 sin 1012 cos 10

10

u w dv w dw

du wdw v w

= =

= = −


( ) ( ) ( )2

2 1sin 10 cos 10 cos 1010 5ww w dw w w w dw= − +∫ ∫

In this example, unlike the previous examples, the new integral will also require integration by parts. For this second integral we will use the following choices.


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( )

( )

cos 101 sin 10

10

u w dv w dw

du dw v w

= =

= =

So, the integral becomes,

( ) ( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

22

2

2

1 1sin 10 cos 10 sin 10 sin 1010 5 10 10

1 1cos 10 sin 10 cos 1010 5 10 100

1cos 10 sin 10 cos 1010 50 500

w ww w dw w w w dw

w ww w w c

w ww w w c

= − + −

= − + + +

= − + + +

∫ ∫

Be careful with the coefficient on the integral for the second application of integration by parts. Since the integral is multiplied by 1

5 we need to make sure that the results of actually doing the

integral are also multiplied by 15 . Forgetting to do this is one of the more common mistakes with

integration by parts problems. As this last example has shown us, we will sometimes need more than one application of integration by parts to completely evaluate an integral. This is something that will happen so don’t get excited about it when it does. In this next example we need to acknowledge an important point about integration techniques. Some integrals can be done in using several different techniques. That is the case with the integral in the next example. Example 5 Evaluate the following integral 1x x dx+∫

(a) Using Integration by Parts. [Solution] (b) Using a standard Calculus I substitution. [Solution]

Solution (a) Evaluate using Integration by Parts. First notice that there are no trig functions or exponentials in this integral. While a good many integration by parts integrals will involve trig functions and/or exponentials not all of them will so don’t get too locked into the idea of expecting them to show up. In this case we’ll use the following choices for u and dv.

( )

32

12 13

u x dv x dx

du dx v x

= = +

= = +



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( ) ( )

( ) ( )

3 32 2

3 52 2

2 21 1 13 32 41 13 15

x x dx x x x dx

x x x c

+ = + − +

= + − + +

∫ ∫

[Return to Problems] (b) Evaluate Using a standard Calculus I substitution. Now let’s do the integral with a substitution. We can use the following substitution. 1 1u x x u du dx= + = − = Notice that we’ll actually use the substitution twice, once for the quantity under the square root and once for the x in front of the square root. The integral is then,

( )

( ) ( )

3 12 2

5 32 2

5 32 2

1 1

2 25 32 21 15 3

x x dx u u du

u u du

u u c

x x c

+ = −

= −

= − +

= + − + +

∫ ∫

∫

[Return to Problems] So, we used two different integration techniques in this example and we got two different answers. The obvious question then should be : Did we do something wrong? Actually, we didn’t do anything wrong. We need to remember the following fact from Calculus I. ( ) ( ) ( ) ( )If thenf x g x f x g x c′ ′= = + In other words, if two functions have the same derivative then they will differ by no more than a constant. So, how does this apply to the above problem? First define the following,

( ) ( ) 1f x g x x x′ ′= = + Then we can compute ( )f x and ( )g x by integrating as follows,

( ) ( ) ( ) ( )f x f x dx g x g x dx′ ′= =∫ ∫ We’ll use integration by parts for the first integral and the substitution for the second integral. Then according to the fact ( )f x and ( )g x should differ by no more than a constant. Let’s

verify this and see if this is the case. We can verify that they differ by no more than a constant if


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we take a look at the difference of the two and do a little algebraic manipulation and simplification.

( ) ( ) ( ) ( )

( ) ( ) ( )

( ) ( )

3 5 5 32 2 2 2

32

32

2 4 2 21 1 1 13 15 5 3

2 4 2 21 1 13 15 5 3

1 00

x x x x x

x x x x

x

+ − + − + − +

= + − + − + +

= +

=

So, in this case it turns out the two functions are exactly the same function since the difference is zero. Note that this won’t always happen. Sometimes the difference will yield a nonzero constant. For an example of this check out the Constant of Integration section in my Calculus I notes. So just what have we learned? First, there will, on occasion, be more than one method for evaluating an integral. Secondly, we saw that different methods will often lead to different answers. Last, even though the answers are different it can be shown, sometimes with a lot of work, that they differ by no more than a constant. When we are faced with an integral the first thing that we’ll need to decide is if there is more than one way to do the integral. If there is more than one way we’ll then need to determine which method we should use. The general rule of thumb that I use in my classes is that you should use the method that you find easiest. This may not be the method that others find easiest, but that doesn’t make it the wrong method. One of the more common mistakes with integration by parts is for people to get too locked into perceived patterns. For instance, all of the previous examples used the basic pattern of taking u to be the polynomial that sat in front of another function and then letting dv be the other function. This will not always happen so we need to be careful and not get locked into any patterns that we think we see. Let’s take a look at some integrals that don’t fit into the above pattern. Example 6 Evaluate the following integral. ln x dx∫ Solution So, unlike any of the other integral we’ve done to this point there is only a single function in the integral and no polynomial sitting in front of the logarithm. The first choice of many people here is to try and fit this into the pattern from above and make the following choices for u and dv.


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1 lnu dv x dx= = This leads to a real problem however since that means v must be,

lnv x dx= ∫ In other words, we would need to know the answer ahead of time in order to actually do the problem. So, this choice simply won’t work. Also notice that with this choice we’d get that

0du = which also causes problems and is another reason why this choice will not work. Therefore, if the logarithm doesn’t belong in the dv it must belong instead in the u. So, let’s use the following choices instead

ln

1u x dv dx

du dx v xx

= =

= =


1ln ln

ln

ln

x dx x x x dxx

x x dx

x x x c

= −

= −

= − +

⌠⌡∫

∫

Example 7 Evaluate the following integral.

5 3 1x x dx+∫ Solution So, if we again try to use the pattern from the first few examples for this integral our choices for u and dv would probably be the following.

5 3 1u x dv x dx= = + However, as with the previous example this won’t work since we can’t easily compute v.

3 1v x dx= +∫ This is not an easy integral to do. However, notice that if we had an x2 in the integral along with the root we could very easily do the integral with a substitution. Also notice that we do have a lot of x’s floating around in the original integral. So instead of putting all the x’s (outside of the root) in the u let’s split them up as follows.

( )

3 2 3

32 3 2

123 19

u x dv x x dx

du x dx v x

= = +

= = +

We can now easily compute v and after using integration by parts we get,


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( ) ( )

( ) ( )

3 35 3 3 3 2 32 2

3 53 3 32 2

2 21 1 19 32 41 19 45

x x dx x x x x dx

x x x c

+ = + − +

= + − + +

⌠⌡∫

So, in the previous two examples we saw cases that didn’t quite fit into any perceived pattern that we might have gotten from the first couple of examples. This is always something that we need to be on the lookout for with integration by parts. Let’s take a look at another example that also illustrates another integration technique that sometimes arises out of integration by parts problems. Example 8 Evaluate the following integral. cos dθ θ θ∫e Solution Okay, to this point we’ve always picked u in such a way that upon differentiating it would make that portion go away or at the very least put it the integral into a form that would make it easier to deal with. In this case no matter which part we make u it will never go away in the differentiation process. It doesn’t much matter which we choose to be u so we’ll choose in the following way. Note however that we could choose the other way as well and we’ll get the same result in the end.

cos

sinu dv ddu d v

θ

θ

θ θ

θ θ

= =

= − =

ee


cos cos sind dθ θ θθ θ θ θ θ= +∫ ∫e e e So, it looks like we’ll do integration by parts again. Here are our choices this time.

sin

cosu dv ddu d v

θ

θ

θ θ

θ θ

= =

= =

ee


cos cos sin cosd dθ θ θ θθ θ θ θ θ θ= + −∫ ∫e e e e Now, at this point it looks like we’re just running in circles. However, notice that we now have the same integral on both sides and on the right side it’s got a minus sign in front of it. This means that we can add the integral to both sides to get,

2 cos cos sindθ θ θθ θ θ θ= +∫e e e


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All we need to do now is divide by 2 and we’re done. The integral is,

( )1cos cos sin2

d cθ θ θθ θ θ θ= + +∫e e e

Notice that after dividing by the two we add in the constant of integration at that point. This idea of integrating until you get the same integral on both sides of the equal sign and then simply solving for the integral is kind of nice to remember. It doesn’t show up all that often, but when it does it may be the only way to actually do the integral. We’ve got one more example to do. As we will see some problems could require us to do integration by parts numerous times and there is a short hand method that will allow us to do multiple applications of integration by parts quickly and easily. Example 9 Evaluate the following integral.

4 2x

x dx∫ e Solution We start off by choosing u and dv as we always would. However, instead of computing du and v we put these into the following table. We then differentiate down the column corresponding to u until we hit zero. In the column corresponding to dv we integrate once for each entry in the first column. There is also a third column which we will explain in a bit and it always starts with a “+” and then alternates signs as shown.

Now, multiply along the diagonals shown in the table. In front of each product put the sign in the third column that corresponds to the “u” term for that product. In this case this would give,

( ) ( ) ( ) ( ) ( )4 4 3 22 2 2 2 2 2

4 3 22 2 2 2 2

2 4 4 12 8 24 16 24 32

2 16 96 384 768

x x x x x x

x x x x x

x dx x x x x

x x x x c

= − + − +

= − + − + +

∫ e e e e e e

e e e e e


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We’ve got the integral. This is much easier than writing down all the various u’s and dv’s that we’d have to do otherwise. So, in this section we’ve seen how to do integration by parts. In your later math classes this is liable to be one of the more frequent integration techniques that you’ll encounter. It is important to not get too locked into patterns that you may think you’ve seen. In most cases any pattern that you think you’ve seen can (and will be) violated at some point in time. Be careful! Also, don’t forget the shorthand method for multiple applications of integration by parts problems. It can save you a fair amount of work on occasion.


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Integrals Involving Trig Functions In this section we are going to look at quite a few integrals involving trig functions and some of the techniques we can use to help us evaluate them. Let’s start off with an integral that we should already be able to do.

5 5

6

cos sin using the substitution sin

1 sin6

x x dx u du u x

x c

= =

= +

∫ ∫

This integral is easy to do with a substitution because the presence of the cosine, however, what about the following integral. Example 1 Evaluate the following integral. 5sin x dx∫ Solution This integral no longer has the cosine in it that would allow us to use the substitution that we used above. Therefore, that substitution won’t work and we are going to have to find another way of doing this integral. Let’s first notice that we could write the integral as follows,

( )25 4 2sin sin sin sin sinx dx x x dx x x dx= =∫ ∫ ∫ Now recall the trig identity, 2 2 2 2cos sin 1 sin 1 cosx x x x+ = ⇒ = − With this identity the integral can be written as,

( )25 2sin 1 cos sinx dx x x dx= −∫ ∫ and we can now use the substitution cosu x= . Doing this gives us,

( )25 2

2 4

3 5

3 5

sin 1

1 2

2 13 5

2 1cos cos cos3 5

x dx u du

u u du

u u u c

x x x c

= − −

= − − +

= − − + +

= − + − +

∫ ∫∫

So, with a little rewriting on the integrand we were able to reduce this to a fairly simple


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substitution. Notice that we were able to do the rewrite that we did in the previous example because the exponent on the sine was odd. In these cases all that we need to do is strip out one of the sines. The exponent on the remaining sines will then be even and we can easily convert the remaining sines to cosines using the identity, 2 2cos sin 1x x+ = (1) If the exponent on the sines had been even this would have been difficult to do. We could strip out a sine, but the remaining sines would then have an odd exponent and while we could convert them to cosines the resulting integral would often be even more difficult than the original integral in most cases. Let’s take a look at another example. Example 2 Evaluate the following integral. 6 3sin cosx x dx∫ Solution So, in this case we’ve got both sines and cosines in the problem and in this case the exponent on the sine is even while the exponent on the cosine is odd. So, we can use a similar technique in this integral. This time we’ll strip out a cosine and convert the rest to sines.

( )( )

6 3 6 2

6 2

6 2

6 8

7 9

sin cos sin cos cos

sin 1 sin cos sin

1

1 1sin sin7 9

x x dx x x x dx

x x x dx u x

u u du

u u du

x x c

=

= − =

= −

= −

= − +

∫ ∫∫∫∫

At this point let’s pause for a second to summarize what we’ve learned so far about integrating powers of sine and cosine.

sin cosn mx x dx∫ (2) In this integral if the exponent on the sines (n) is odd we can strip out one sine, convert the rest to cosines using (1) and then use the substitution cosu x= . Likewise, if the exponent on the cosines (m) is odd we can strip out one cosine and convert the rest to sines and the use the substitution sinu x= . Of course, if both exponents are odd then we can use either method. However, in these cases it’s usually easier to convert the term with the smaller exponent.


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The one case we haven’t looked at is what happens if both of the exponents are even? In this case the technique we used in the first couple of examples simply won’t work and in fact there really isn’t any one set method for doing these integrals. Each integral is different and in some cases there will be more than one way to do the integral. With that being said most, if not all, of integrals involving products of sines and cosines in which both exponents are even can be done using one or more of the following formulas to rewrite the integrand.

( )( )

( )( )

( )

2

2

1cos 1 cos 221sin 1 cos 22

1sin cos sin 22

x x

x x

x x x

= +

= −

=

The first two formulas are the standard half angle formula from a trig class written in a form that will be more convenient for us to use. The last is the standard double angle formula for sine, again with a small rewrite. Let’s take a look at an example. Example 3 Evaluate the following integral. 2 2sin cosx x dx∫ Solution As noted above there are often more than one way to do integrals in which both of the exponents are even. This integral is an example of that. There are at least two solution techniques for this problem. We will do both solutions starting with what is probably the harder of the two, but it’s also the one that many people see first. Solution 1 In this solution we will use the two half angle formulas above and just substitute them into the integral.

( )( ) ( )( )

( )

2 2

2

1 1sin cos 1 cos 2 1 cos 22 2

1 1 cos 24

x x dx x x dx

x dx

= − +

= −

⌠⌡∫

∫

So, we still have an integral that can’t be completely done, however notice that we have managed to reduce the integral down to just one term causing problems (a cosine with an even power) rather than two terms causing problems. In fact to eliminate the remaining problem term all that we need to do is reuse the first half angle


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formula given above.

( )( )

( )

( )

( )

2 2 1 1sin cos 1 1 cos 44 21 1 1 cos 44 2 21 1 1 sin 44 2 81 1 sin 48 32

x x dx x dx

x dx

x x c

x x c

= − +

= −

= − +

= − +

⌠⌡

⌠⌡

∫

So, this solution required a total of three trig identities to complete. Solution 2 In this solution we will use the half angle formula to help simplify the integral as follows.

( )

( )

( )

22 2

2

2

sin cos sin cos

1 sin 22

1 sin 24

x x dx x x dx

x dx

x dx

=

=

=

⌠⌡

∫ ∫

∫

Now, we use the double angle formula for sine to reduce to an integral that we can do.

( )

( )

2 2 1sin cos 1 cos 481 1 sin 48 32

x x dx x dx

x x c

= −

= − +

∫ ∫

This method required only two trig identities to complete. Notice that the difference between these two methods is more one of “messiness”. The second method is not appreciably easier (other than needing one less trig identity) it is just not as messy and that will often translate into an “easier” process. In the previous example we saw two different solution methods that gave the same answer. Note that this will not always happen. In fact, more often than not we will get different answers. However, as we discussed in the Integration by Parts section, the two answers will differ by no more than a constant. In general when we have products of sines and cosines in which both exponents are even we will need to use a series of half angle and/or double angle formulas to reduce the integral into a form that we can integrate. Also, the larger the exponents the more we’ll need to use these formulas and hence the messier the problem.


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Sometimes in the process of reducing integrals in which both exponents are even we will run across products of sine and cosine in which the arguments are different. These will require one of the following formulas to reduce the products to integrals that we can do.

( ) ( )

( ) ( )

( ) ( )

1sin cos sin sin21sin sin cos cos21cos cos cos cos2

α β α β α β

α β α β α β

α β α β α β

= − + +

= − − +

= − + +

Let’s take a look at an example of one of these kinds of integrals. Example 4 Evaluate the following integral. ( ) ( )cos 15 cos 4x x dx∫ Solution This integral requires the last formula listed above.

( ) ( ) ( ) ( )

( ) ( )

1cos 15 cos 4 cos 11 cos 1921 1 1sin 11 sin 192 11 19

x x dx x x dx

x x c

= +

= + +

∫ ∫

Okay, at this point we’ve covered pretty much all the possible cases involving products of sines and cosines. It’s now time to look at integrals that involve products of secants and tangents. This time, let’s do a little analysis of the possibilities before we just jump into examples. The general integral will be,

sec tann mx x dx∫ (3) The first thing to notice is that we can easily convert even powers of secants to tangents and even powers of tangents to secants by using a formula similar to (1). In fact, the formula can be derived from (1) so let’s do that.

2 2

2 2

2 2 2

sin cos 1sin cos 1cos cos cos

x xx xx x x

+ =

+ =


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2 2tan 1 secx x+ = (4) Now, we’re going to want to deal with (3) similarly to how we dealt with (2). We’ll want to eventually use one of the following substitutions.

2tan sec

sec sec tanu x du x dxu x du x x dx

= == =

So, if we use the substitution tanu x= we will need two secants left for the substitution to work. This means that if the exponent on the secant (n) is even we can strip two out and then convert the remaining secants to tangents using (4). Next, if we want to use the substitution secu x= we will need one secant and one tangent left over in order to use the substitution. This means that if the exponent on the tangent (m) is odd and we have at least one secant in the integrand we can strip out one of the tangents along with one of the secants of course. The tangent will then have an even exponent and so we can use (4) to convert the rest of the tangents to secants. Note that this method does require that we have at least one secant in the integral as well. If there aren’t any secants then we’ll need to do something different. If the exponent on the secant is even and the exponent on the tangent is odd then we can use either case. Again, it will be easier to convert the term with the smallest exponent. Let’s take a look at a couple of examples. Example 5 Evaluate the following integral. 9 5sec tanx x dx∫ Solution First note that since the exponent on the secant isn’t even we can’t use the substitution tanu x= . However, the exponent on the tangent is odd and we’ve got a secant in the integral and so we will be able to use the substitution secu x= . This means striping out a single tangent (along with a secant) and converting the remaining tangents to secants using (4). Here’s the work for this integral.

( )( )

9 5 8 4

28 2

28 2

12 10 8

13 11 9

sec tan sec tan tan sec

sec sec 1 tan sec sec

1

2

1 2 1sec sec sec13 11 9

x x dx x x x x dx

x x x x dx u x

u u du

u u u du

x x x c

=

= − =

= −

= − +

= − + +

∫ ∫∫∫∫


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Example 6 Evaluate the following integral. 4 6sec tanx x dx∫ Solution So, in this example the exponent on the tangent is even so the substitution secu x= won’t work. The exponent on the secant is even and so we can use the substitution tanu x= for this integral. That means that we need to strip out two secants and convert the rest to tangents. Here is the work for this integral.

( )( )

4 6 2 6 2

2 6 2

2 6

8 6

9 7

sec tan sec tan sec

tan 1 tan sec tan

1

1 1tan tan9 7

x x dx x x x dx

x x x dx u x

u u du

u u du

x x c

=

= + =

= +

= +

= + +

∫ ∫∫∫∫

Both of the previous examples fit very nicely into the patterns discussed above and so were not all that difficult to work. However, there are a couple of exceptions to the patterns above and in these cases there is no single method that will work for every problem. Each integral will be different and may require different solution methods in order to evaluate the integral. Let’s first take a look at a couple of integrals that have odd exponents on the tangents, but no secants. In these cases we can’t use the substitution secu x= since it requires there to be at least one secant in the integral. Example 7 Evaluate the following integral. tan x dx∫ Solution To do this integral all we need to do is recall the definition of tangent in terms of sine and cosine and then this integral is nothing more than a Calculus I substitution.

1

sintan coscos

1

ln cos ln ln

ln cos

ln sec

r

xx dx dx u xx

duu

x c r x x

x c

x c

−

= =

= −

= − + =

= +

+

⌠⌡

⌠⌡

∫

Example 8 Evaluate the following integral. 3tan x dx∫


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Solution The trick to this one is do the following manipulation of the integrand.

( )

3 2

2

2

tan tan tan

tan sec 1

tan sec tan

x dx x x dx

x x dx

x x dx x dx

=

= −

= −

∫ ∫∫∫ ∫

We can now use the substitution tanu x= on the first integral and the results from the previous example to on the second integral. The integral is then,

3 21tan tan ln sec2

x dx x x c= − +∫

Note that all odd powers of tangent (with the exception of the first power) can be integrated using the same method we used in the previous example. For instance,

( )5 3 2 3 2 3tan tan sec 1 tan sec tanx dx x x dx x x dx x dx= − = −∫ ∫ ∫ ∫ So, a quick substitution ( tanu x= ) will give us the first integral and the second integral will always be the previous odd power. Now let’s take a look at a couple of examples in which the exponent on the secant is odd and the exponent on the tangent is even. In these cases the substitutions used above won’t work. It should also be noted that both of the following two integrals are integrals that we’ll be seeing on occasion in later sections of this chapter and in later chapters. Because of this it wouldn’t be a bad idea to make a note of these results so you’ll have them ready when you need them later. Example 9 Evaluate the following integral. sec x dx∫ Solution This one isn’t too bad once you see what you’ve got to do. By itself the integral can’t be done. However, if we manipulate the integrand as follows we can do it.

( )

2

sec sec tansec

sec tansec tan sec

sec tan

x x xx dx dx

x xx x x dx

x x

+=

++

=+

⌠⌡

⌠⌡

∫

In this form we can do the integral using the substitution sec tanu x x= + . Doing this gives,

sec ln sec tanx dx x x c= + +∫


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The idea used in the above example is a nice idea to keep in mind. Multiplying the numerator and denominator of a term by the same term above can, on occasion, put the integral into a form that can be integrated. Note that this method won’t always work and even when it does it won’t always be clear what you need to multiply the numerator and denominator by. However, when it does work and you can figure out what term you need it can greatly simplify the integral. Here’s the next example. Example 10 Evaluate the following integral. 3sec x dx∫ Solution This one is different from any of the other integrals that we’ve done in this section. The first step to doing this integral is to perform integration by parts using the following choices for u and dv.

2sec sec

sec tan tanu x dv x dxdu x x dx v x

= == =

Note that using integration by parts on this problem is not an obvious choice, but it does work very nicely here. After doing integration by parts we have,

3 2sec sec tan sec tanx dx x x x x dx= −∫ ∫ Now the new integral also has an odd exponent on the secant and an even exponent on the tangent and so the previous examples of products of secants and tangents still won’t do us any good. To do this integral we’ll first write the tangents in the integral in terms of secants. Again, this is not necessarily an obvious choice but it’s what we need to do in this case.

( )3 2

3

sec sec tan sec sec 1

sec tan sec sec

x dx x x x x dx

x x x dx x dx

= − −

= − +

∫ ∫∫ ∫

Now, we can use the results from the previous example to do the second integral and notice that the first integral is exactly the integral we’re being asked to evaluate with a minus sign in front. So, add it to both sides to get,

32 sec sec tan ln sec tanx dx x x x x= + +∫ Finally divide by a two and we’re done.

( )3 1sec sec tan ln sec tan2

x dx x x x x c= + + +∫

Again, note that we’ve again used the idea of integrating the right side until the original integral shows up and then moving this to the left side and dividing by its coefficient to complete the evaluation. We first saw this in the Integration by Parts section and noted at the time that this was a nice technique to remember. Here is another example of this technique.


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Now that we’ve looked at products of secants and tangents let’s also acknowledge that because we can relate cosecants and cotangents by 2 21 cot cscx x+ = all of the work that we did for products of secants and tangents will also work for products of cosecants and cotangents. We’ll leave it to you to verify that. There is one final topic to be discussed in this section before moving on. To this point we’ve looked only at products of sines and cosines and products of secants and tangents. However, the methods used to do these integrals can also be used on some quotients involving sines and cosines and quotients involving secants and tangents (and hence quotients involving cosecants and cotangents). Let’s take a quick look at an example of this. Example 11 Evaluate the following integral.

7

4

sincos

x dxx

⌠⌡

Solution If this were a product of sines and cosines we would know what to do. We would strip out a sine (since the exponent on the sine is odd) and convert the rest of the sines to cosines. The same idea will work in this case. We’ll strip out a sine from the numerator and convert the rest to cosines as follows,

( )

( )

7 6

4 4

32

4

32

4

sin sin sincos cos

sinsin

cos

1 cossin

cos

x xdx x dxx x

xx dx

x

xx dx

x

=

=

−=

⌠ ⌠ ⌡ ⌡

⌠⌡

⌠⌡

At this point all we need to do is use the substitution cosu x= and we’re done.

( )327

4 4

4 2 2

33

33

1sincos

3 3

1 1 1 13 33 3

1 3 13cos cos3cos cos 3

ux dx dux u

u u u du

u u cu u

x x cx x

− −

−= −

= − − + −

= − − + + − +

= − − + +

⌠⌠ ⌡ ⌡

∫


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So, under the right circumstances, we can use the ideas developed to help us deal with products of trig functions to deal with quotients of trig functions. The natural question then, is just what are the right circumstances? First notice that if the quotient had been reversed as in this integral,

4

7

cossin

x dxx

⌠⌡

we wouldn’t have been able to strip out a sine.

4 4

7 6

cos cos 1sin sin sin

x xdx dxx x x

=⌠ ⌠ ⌡ ⌡

In this case the “stripped out” sine remains in the denominator and it won’t do us any good for the substitution cosu x= since this substitution requires a sine in the numerator of the quotient. Also note that, while we could convert the sines to cosines, the resulting integral would still be a fairly difficult integral. So, we can use the methods we applied to products of trig functions to quotients of trig functions provided the term that needs parts stripped out in is the numerator of the quotient.


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Trig Substitutions As we have done in the last couple of sections, let’s start off with a couple of integrals that we should already be able to do with a standard substitution.

( )3

2 2 222

1 125 4 25 4 25 475 2525 4

xx x dx x c dx x cx

− = − + = − +−

⌠⌡∫

Both of these used the substitution 225 4u x= − and at this point should be pretty easy for you to do. However, let’s take a look at the following integral. Example 1 Evaluate the following integral.

225 4x dx

x−⌠

⌡

Solution In this case the substitution 225 4u x= − will not work and so we’re going to have to do something different for this integral. It would be nice if we could get rid of the square root somehow. The following substitution will do that for us.

2 sec5

x θ=

Do not worry about where this came from at this point. As we work the problem you will see that it works and that if we have a similar type of square root in the problem we can always use a similar substitution. Before we actually do the substitution however let’s verify the claim that this will allow us to get rid of the square root.

( )2 2 2 2425 4 25 sec 4 4 sec 1 2 sec 125

x θ θ θ − = − = − = −

To get rid of the square root all we need to do is recall the relationship, 2 2 2 2tan 1 sec sec 1 tanθ θ θ θ+ = ⇒ − = Using this fact the square root becomes,

2 225 4 2 tan 2 tanx θ θ− = = Note the presence of the absolute value bars there. These are important. Recall that

2x x= There should always be absolute value bars at this stage. If we knew that tanθ was always


Math 140


positive or always negative we could eliminate the absolute value bars using,

if 0if 0

x xx

x x≥

= − <

Without limits we won’t be able to determine if tanθ is positive or negative, however, we will need to eliminate them in order to do the integral. Therefore, since we are doing an indefinite integral we will assume that tanθ will be positive and so we can drop the absolute value bars. This gives,

225 4 2 tanx θ− = So, we were able to eliminate the square root using this substitution. Let’s now do the substitution and see what we get. In doing the substitution don’t forget that we’ll also need to substitute for the dx. This is easy enough to get from the substitution.

2 2sec sec tan5 5

x dx dθ θ θ θ= ⇒ =

Using this substitution the integral becomes,

2

25

2

25 4 2 tan 2 sec tansec 5

2 tan

x dx dx

d

θθ θ θ

θ

θ θ

− =

=

⌠ ⌠⌡⌡

∫

With this substitution we were able to reduce the given integral to an integral involving trig functions and we saw how to do these problems in the previous section. Let’s finish the integral.

( )

2225 4 2 sec 1

2 tan

x dx dx

c

θ θ

θ θ

−= −

= − +

⌠⌡ ∫

So, we’ve got an answer for the integral. Unfortunately the answer isn’t given in x’s as it should be. So, we need to write our answer in terms of x. We can do this with some right triangle trig. From our original substitution we have,

5 hypotenusesec2 adjacentx

θ = =

This gives the following right triangle.


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From this we can see that,

225 4tan

2x

θ−

=

We can deal with the θ in one of any variety of ways. From our substitution we can see that,

1 5sec2x

θ − =

While this is a perfectly acceptable method of dealing with the θ we can use any of the possible six inverse trig functions and since sine and cosine are the two trig functions most people are familiar with we will usually use the inverse sine or inverse cosine. In this case we’ll use the inverse cosine.

1 2cos5x

θ − =

So, with all of this the integral becomes,

2 21

2 1

25 4 25 4 22 cos2 5

225 4 2cos5

x xdx cx x

x cx

−

−

− − = − + = − − +

⌠⌡

We now have the answer back in terms of x. Wow! That was a lot of work. Most of these won’t take as long to work however. This first one needed lot’s of explanation since it was the first one. The remaining examples won’t need quite as much explanation and so won’t take as long to work. However, before we move onto more problems let’s first address the issue of definite integrals and how the process differs in these cases. Example 2 Evaluate the following integral.

4

25

25

25 4x dxx

−⌠⌡


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Solution The limits here won’t change the substitution so that will remain the same.

2 sec5

x θ=

Using this substitution the square root still reduces down to,

225 4 2 tanx θ− = However, unlike the previous example we can’t just drop the absolute value bars. In this case we’ve got limits on the integral and so we can use the limits as well as the substitution to determine the range of θ that we’re in. Once we’ve got that we can determine how to drop the absolute value bars. Here’s the limits of θ .

2 2 2 sec 05 5 54 4 2 sec5 5 5 3

x

x

θ θ

πθ θ

= ⇒ = ⇒ =

= ⇒ = ⇒ =

So, if we are in the range 2 4

5 5x≤ ≤ then θ is in the range of 30 πθ≤ ≤ and in this range of θ ’s tangent is positive and so we can just drop the absolute value bars. Let’s do the substitution. Note that the work is identical to the previous example and so most of it is left out. We’ll pick up at the final integral and then do the substitution.

( )

425 23

025

3

0

25 4 2 sec 1

2 tan

22 33

x dx dx

π

π

θ θ

θ θ

π

−= −

= −

= −

⌠⌡ ∫

Note that because of the limits we didn’t need to resort to a right triangle to complete the problem. Let’s take a look at a different set of limits for this integral. Example 3 Evaluate the following integral.

2

25

45

25 4x dxx

−

−

−⌠⌡

Solution Again, the substitution and square root are the same as the first two examples.


Math 140


22 sec 25 4 2 tan5

x xθ θ= − =

Let’s next see the limits θ for this problem.

2 2 2 sec5 5 54 4 2 2sec5 5 5 3

x

x

θ θ π

πθ θ

= − ⇒ − = ⇒ =

= − ⇒ − = ⇒ =

Note that in determining the value of θ we used the smallest positive value. Now for this range of x’s we have 2

3π θ π≤ ≤ and in this range of θ tangent is negative and so in this case we can

drop the absolute value bars, but will need to add in a minus sign upon doing so. In other words,

225 4 2 tanx θ− = − So, the only change this will make in the integration process is to put a minus sign in front of the integral. The integral is then,

( )

225 2

24 35

2 3

25 4 2 sec 1

2 tan

2 2 33

x dx dx

π

π

π

π

θ θ

θ θ

π

−

−

−= − −

= − −

= −

⌠⌡ ∫

In the last two examples we saw that we have to be very careful with definite integrals. We need to make sure that we determine the limits on θ and whether or not this will mean that we can drop the absolute value bars or if we need to add in a minus sign when we drop them. Before moving on to the next example let’s get the general form for the substitution that we used in the previous set of examples.

2 2 2 secab x a xb

θ− ⇒ =

Let’s work a new and different type of example. Example 4 Evaluate the following integral.

4 2

19

dxx x−

⌠⌡

Solution Now, the square root in this problem looks to be (almost) the same as the previous ones so let’s try the same type of substitution and see if it will work here as well.


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3secx θ= Using this substitution the square root becomes,

2 2 2 29 9 9sec 3 1 sec 3 tanx θ θ θ− = − = − = − So using this substitution we will end up with a negative quantity (the tangent squared is always positive of course) under the square root and this will be trouble. Using this substitution will give complex values and we don’t want that. So, using secant for the substitution won’t work. However, the following substitution (and differential) will work.

3sin 3cosx dx dθ θ θ= = With this substitution the square root is,

2 2 29 3 1 sin 3 cos 3 cos 3cosx θ θ θ θ− = − = = = We were able to drop the absolute value bars because we are doing an indefinite integral and so we’ll assume that everything is positive. The integral is now,

( )44 2

4

4

1 1 3cos81sin 3cos9

1 181 sin1 csc81

dx dx x

d

d

θ θθ θ

θθ

θ θ

=−

=

=

⌠⌠ ⌡ ⌡

⌠⌡

∫

In the previous section we saw how to deal with integrals in which the exponent on the secant was even and since cosecants behave an awful lot like secants we should be able to do something similar with this. Here is the integral.

( )

2 2

4 2

2 2

2

3

1 1 csc csc8191 cot 1 csc cot81

1 1811 1 cot cot81 3

dx dx x

d u

u du

c

θ θ θ

θ θ θ θ

θ θ

=−

= + =

= − +

= − + +

⌠⌡ ∫

∫

∫

Now we need to go back to x’s using a right triangle. Here is the right triangle for this problem


Math 140


and trig functions for this problem.

29sin cot

3x x

xθ θ

−= =


( )

32 2

4 2

32 2 2

3

1 1 1 9 981 39

9 9243 81

x xdx cx xx x

x x cx x

− − = − + + −

− −= − − +

⌠⌡

Here’s the general form for this type of square root.

2 2 2 sinaa b x xb

θ− ⇒ =

There is one final case that we need to look at. The next integral will also contain something that we need to make sure we can deal with. Example 5 Evaluate the following integral.

( )

16 5

32 20 36 1

x dxx +

⌠⌡

Solution First, notice that there really is a square root in this problem even though it isn’t explicitly written out. To see the root let’s rewrite things a little.

( ) ( ) ( )33 1 3

2 2 22 236 1 36 1 36 1x x x

+ = + = +

This square root is not in the form we saw in the previous examples. Here we will use the substitution for this root.

21 1tan sec6 6

x dx dθ θ θ= =


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With this substitution the denominator becomes,

( ) ( ) ( )3 3 3 32 2 236 1 tan 1 sec secx θ θ θ+ = + = =

Now, because we have limits we’ll need to convert them to θ so we can determine how to drop the absolute value bars.

10 0 tan 06

1 1 1 tan6 6 6 4

x

x

θ θ

πθ θ

= ⇒ = ⇒ =

= ⇒ = ⇒ =

In this range of θ secant is positive and so we can drop the absolute value bars. Here is the integral,

( )

16 55 14 27776

3 32 020

54

0

tan 1 secsec 636 1

1 tan46656 sec

x dx dx

d

π

π

θθ θ

θ

θθ

θ

= +

=

⌠ ⌠ ⌡⌡

⌠⌡

There are several ways to proceed from this point. Normally with an odd exponent on the tangent we would strip one of them out and convert to secants. However, that would require that we also have a secant in the numerator which we don’t have. Therefore, it seems like the best way to do this one would be to convert the integrand to sines and cosines.

( )( )

16 5 54

3 42 020

224

40

1 sin46656 cos36 1

1 cos1 sin46656 cos

x dx dx

d

π

π

θθ

θ

θθ θ

θ

=+

−=

⌠ ⌠ ⌡⌡

⌠⌡

We can now use the substitution cosu θ= and we might as well convert the limits as well.

0 cos 0 1

2cos4 4 2

u

u

θ

π πθ

= = =

= = =



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( )

16 25

4 223 1

2 20

22

31

1 2 14665636 1

1 1 246656 3

1 11 217496 279936

x dx u u dux

uu u

− −= − − ++

= − − + +

= −

⌠⌡

∫

The general form for this final type of square root is

2 2 2 tanaa b x xb

θ+ ⇒ =

We have a couple of final examples to work in this section. Not all trig substitutions will just jump right out at us. Sometimes we need to do a little work on the integrand first to get it into the correct form and that is the point of the remaining examples. Example 6 Evaluate the following integral.

22 4 7

x dxx x− −

⌠⌡

Solution In this case the quantity under the root doesn’t obviously fit into any of the cases we looked at above and in fact isn’t in the any of the forms we saw in the previous examples. Note however that if we complete the square on the quadratic we can make it look somewhat like the above integrals. Remember that completing the square requires a coefficient of one in front of the x2. Once we have that we take half the coefficient of the x, square it, and then add and subtract it to the quantity. Here is the completing the square for this problem.

( ) ( )2 22 27 7 92 2 2 2 1 1 2 1 2 1 92 2 2

x x x x x x − − = − + − − = − − = − −

So, the root becomes,

( )222 4 7 2 1 9x x x− − = − − This looks like a secant substitution except we don’t just have an x that is squared. That is okay, it will work the same way.

3 3 31 sec 1 sec sec tan2 2 2

x x dx dθ θ θ θ θ− = = + =


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Using this substitution the root reduces to,

( )22 2 22 4 7 2 1 9 9sec 9 3 tan 3 tan 3tanx x x θ θ θ θ− − = − − = − = = = Note we could drop the absolute value bars since we are doing an indefinite integral. Here is the integral.

32

2

2

1 sec 3 sec tan3tan 22 4 7

1 3sec sec22

1 3ln sec tan tan22

x dx dx x

d

c

θθ θ θ

θ

θ θ θ

θ θ θ

+ = − −

= +

= + + +

⌠⌠ ⌡ ⌡

⌠⌡

And here is the right triangle for this problem.

( ) 22 1 2 4 7sec tan3 3x x x

θ θ− − −

= =


( ) 2 2

2

2 11 2 4 7 2 4 7ln3 3 222 4 7

xx x x x xdx cx x

− − − − −= + + +

− −

⌠⌡


4 21x x dx+∫e e Solution This doesn’t look to be anything like the other problems in this section. However it is. To see this we first need to notice that,

( )22x x=e e With this we can use the following substitution.


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2tan secx x dx dθ θ θ= =e e Remember that to compute the differential all we do is differentiate both sides and then tack on dx or dθ onto the appropriate side. With this substitution the square root becomes,

( )22 2 21 1 1 tan sec sec secx x θ θ θ θ+ = + = + = = =e e Again, we can drop the absolute value bars because we are doing an indefinite integral. Here’s the integral.

( ) ( )( )( )

( )

4 2 3 2

3 2

3 2

2 2

4 2

5 3

1 1

1

tan sec sec

sec 1 sec sec tan sec

1 1sec sec5 3

x x x x x

x x x

dx dx

dx

d

d u

u u du

c

θ θ θ θ

θ θ θ θ θ θ

θ θ

+ = +

= +

=

= − =

= −

= − +

∫ ∫∫∫∫∫

e e e e e

e e e

Here is the right triangle for this integral.

2

21tan sec 11 1

x xxθ θ

+= = = +

e e e


( ) ( )5 3

4 2 2 22 21 11 1 15 3

x x x xdx c+ = + − + +∫e e e e

So, as we’ve seen in the final two examples in this section some integrals that look nothing like the first few examples can in fact be turned into a trig substitution problem with a little work. Before leaving this section let’s summarize all three cases in one place.


Math 140


2 2 2

2 2 2

2 2 2

sin

sec

tan

aa b x xbab x a xbaa b x xb

θ

θ

θ

− ⇒ =

− ⇒ =

+ ⇒ =


Math 140


Partial Fractions In this section we are going to take a look at integrals of rational expressions of polynomials and once again let’s start this section out with an integral that we can already do so we can contrast it with the integrals that we’ll be doing in this section.

( )2

2

2

2 1 1 using 6 and 2 16

ln 6

x dx du u x x du x dxx x u

x x c

−= = − − = −

− −= − − +

⌠ ⌠ ⌡ ⌡

So, if the numerator is the derivative of the denominator (or a constant multiple of the derivative of the denominator) doing this kind of integral is fairly simple. However, often the numerator isn’t the derivative of the denominator (or a constant multiple). For example, consider the following integral.

2

3 116

x dxx x

+− −

⌠⌡

In this case the numerator is definitely not the derivative of the denominator nor is it a constant multiple of the derivative of the denominator. Therefore, the simple substitution that we used above won’t work. However, if we notice that the integrand can be broken up as follows,

2

3 11 4 16 3 2

xx x x x

+= −

− − − +

then the integral is actually quite simple.

2

3 11 4 16 3 2

4ln 3 ln 2

x dx dxx x x x

x x c

+= −

− − − += − − + +

⌠ ⌠ ⌡ ⌡

This process of taking a rational expression and decomposing it into simpler rational expressions that we can add or subtract to get the original rational expression is called partial fraction decomposition. Many integrals involving rational expressions can be done if we first do partial fractions on the integrand. So, let’s do a quick review of partial fractions. We’ll start with a rational expression in the form,

( ) ( )( )

P xf x

Q x=

where both P(x) and Q(x) are polynomials and the degree of P(x) is smaller than the degree of Q(x). Recall that the degree of a polynomial is the largest exponent in the polynomial. Partial fractions can only be done if the degree of the numerator is strictly less than the degree of the denominator. That is important to remember. So, once we’ve determined that partial fractions can be done we factor the denominator as completely as possible. Then for each factor in the denominator we can use the following table to determine the term(s) we pick up in the partial fraction decomposition.


Math 140


Factor in denominator

Term in partial fraction decomposition

ax b+ A

ax b+

( )kax b+ ( ) ( )1 2

2k

kAA A

ax b ax b ax b+ + +

+ + +L , 1,2,3,k = …

2ax bx c+ + 2

Ax Bax bx c

++ +

( )2 kax bx c+ + ( ) ( )

1 1 2 222 2 2

k kk

A x BA x B A x Bax bx c ax bx c ax bx c

++ ++ + +

+ + + + + +L , 1,2,3,k = …

Notice that the first and third cases are really special cases of the second and fourth cases respectively. There are several methods for determining the coefficients for each term and we will go over each of those in the following examples. Let’s start the examples by doing the integral above. Example 1 Evaluate the following integral.

2

3 116

x dxx x

+− −

⌠⌡

Solution The first step is to factor the denominator as much as possible and get the form of the partial fraction decomposition. Doing this gives,

( ) ( )

3 113 2 3 2x A B

x x x x+

= +− + − +

The next step is to actually add the right side back up.

( ) ( )

( ) ( )( )( )

2 33 113 2 3 2

A x B xxx x x x

+ + −+=

− + − +

Now, we need to choose A and B so that the numerators of these two are equal for every x. To do this we’ll need to set the numerators equal. ( ) ( )3 11 2 3x A x B x+ = + + −


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Note that in most problems we will go straight from the general form of the decomposition to this step and not bother with actually adding the terms back up. The only point to adding the terms is to get the numerator and we can get that without actually writing down the results of the addition. At this point we have one of two ways to proceed. One way will always work, but is often more work. The other, while it won’t always work, is often quicker when it does work. In this case both will work and so we’ll use the quicker way for this example. We’ll take a look at the other method in a later example. What we’re going to do here is to notice that the numerators must be equal for any x that we would choose to use. In particular the numerators must be equal for 2x = − and 3x = . So, let’s plug these in and see what we get.

( ) ( )

( ) ( )2 5 0 5 1

3 20 5 0 4

x A B B

x A B A

= − = + − ⇒ = −

= = + ⇒ =

So, by carefully picking the x’s we got the unknown constants to quickly drop out. Note that these are the values we claimed they would be above. At this point there really isn’t a whole lot to do other than the integral.

2

3 11 4 16 3 2

4 13 2

4ln 3 ln 2

x dx dxx x x x

dx dxx x

x x c

+= −

− − − +

= −− +

= − − + +

⌠ ⌠ ⌡ ⌡

⌠ ⌠ ⌡ ⌡

Recall that to do this integral we first split it up into two integrals and then used the substitutions, 3 2u x v x= − = + on the integrals to get the final answer. Before moving onto the next example a couple of quick notes are in order here. First, many of the integrals in partial fractions problems come down to the type of integral seen above. Make sure that you can do those integrals. There is also another integral that often shows up in these kinds of problems so we may as well give the formula for it here since we are already on the subject.

12 2

1 1 tan xdx cx a a a

− = + + ⌠⌡

It will be an example or two before we use this so don’t forget about it. Now, let’s work some more examples.


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2

3 2

43 4 4

x dxx x x

++ −

⌠⌡

Solution We won’t be putting as much detail into this solution as we did in the previous example. The first thing is to factor the denominator and get the form of the partial fraction decomposition.

( )( )

2 42 3 2 2 3 2

x A B Cx x x x x x

+= + +

+ − + −

The next step is to set numerators equal. If you need to actually add the right side together to get the numerator for that side then you should do so, however, it will definitely make the problem quicker if you can do the addition in your head to get, ( ) ( ) ( ) ( )2 4 2 3 2 3 2 2x A x x Bx x Cx x+ = + − + − + + As with the previous example it looks like we can just pick a few values of x and find the constants so let’s do that.

( ) ( )

( )( )

0 4 2 2 112 8 2 82

2 40 2 8 40 53 9 3 3 16 2

x A A

x B B

x C C

= = − ⇒ = −

= − = − − ⇒ =

= = ⇒ = =

Note that unlike the first example most of the coefficients here are fractions. That is not unusual so don’t get excited about it when it happens. Now, let’s do the integral.

2 512 2

3 2

4 13 4 4 2 3 2

1 5ln ln 2 ln 3 22 6

x dx dxx x x x x x

x x x c

+= − + +

+ − + −

= − + + + − +

⌠ ⌠⌡⌡

Again, as noted above, integrals that generate natural logarithms are very common in these problems so make sure you can do them. Example 3 Evaluate the following integral.

( ) ( )

2

2 2

29 54 3

x x dxx x

− +

− +

⌠⌡

Solution


Math 140


This time the denominator is already factored so let’s just jump right to the partial fraction decomposition.

( ) ( ) ( )

2

2 2 22

29 54 34 3 4

x x A B Cx Dx xx x x

− + += + +

− +− + −

Setting numerators gives,

( ) ( ) ( ) ( )( )22 2 229 5 4 3 3 4x x A x x B x Cx D x− + = − + + + + + − In this case we aren’t going to be able to just pick values of x that will give us all the constants. Therefore, we will need to work this the second (and often longer) way. The first step is to multiply out the right side and collect all the like terms together. Doing this gives, ( ) ( ) ( )2 3 229 5 4 8 3 16 8 12 3 16x x A C x A B C D x A C D x A B D− + = + + − + − + + + − − + + Now we need to choose A, B, C, and D so that these two are equal. In other words we will need to set the coefficients of like powers of x equal. This will give a system of equations that can be solved.

3

2

1

0

: 0: 4 8 1

1, 5, 1, 2: 3 16 8 29: 12 3 16 5

x A Cx A B C D

A B C Dx A C Dx A B D

+ =

− + − + = ⇒ = = − = − =+ − = −

− + + =

Note that we used x0 to represent the constants. Also note that these systems can often be quite large and have a fair amount of work involved in solving them. The best way to deal with these is to use some form of computer aided solving techniques. Now, let’s take a look at the integral.

( ) ( ) ( )

( )

2

2 2 22

2 2 2

2 1

29 5 1 5 24 34 3 4

1 5 24 3 34

5 1 2ln 4 ln 3 tan4 2 3 3

x x xdx dxx xx x x

x dxx x xx

xx x cx

−

− + − += − +

− +− + −

= − − +− + +−

= − + − + + + −

⌠ ⌠

⌡⌡

⌠⌡

In order to take care of the third term we needed to split it up into two separate terms. Once we’ve done this we can do all the integrals in the problem. The first two use the substitution

4u x= − , the third uses the substitution 2 3v x= + and the fourth term uses the formula given above for inverse tangents. Example 4 Evaluate the following integral.


Math 140


( )( )3 2

22

10 3 36

1 4

x x x dxx x

+ + +

− +

⌠⌡

Solution Let’s first get the general form of the partial fraction decomposition.

( ) ( ) ( )3 2

2 222 2

10 3 361 41 4 4

x x x A Bx C Dx Ex xx x x

+ + + + += + +

− +− + +

Now, set numerators equal, expand the right side and collect like terms.

( ) ( )( ) ( ) ( )( )( ) ( ) ( )

( )

23 2 2 2

4 3 2

10 3 36 4 1 4 1

8 4

4 4 16 4

x x x A x Bx C x x Dx E x

A B x C B x A B C D x

B C D E x A C E

+ + + = + + + − + + + −

= + + − + + − + +

− + − + + − −

Setting coefficient equal gives the following system.

4

3

2

1

0

: 0: 1: 8 4 10 2, 2, 1, 1, 0: 4 4 3: 16 4 36

x A Bx C Bx A B C D A B C D Ex B C D Ex A C E

+ =

− = + − + = ⇒ = = − = − = =− + − + = − − =

Don’t get excited if some of the coefficients end up being zero. It happens on occasion. Here’s the integral.

( )( ) ( )

( )

3 2

2 222 2

22 2 2

2 12

10 3 36 2 2 11 41 4 4

2 2 11 4 4 4

1 1 12ln 1 ln 4 tan2 2 2 4

x x x x xdx dxx xx x x

x x dxx x x x

xx x cx

−

+ + + − −= + +

− +− + +

= − − +− + + +

= − − + − − + +

⌠ ⌠ ⌡⌡

⌠⌡

To this point we’ve only looked at rational expressions where the degree of the numerator was strictly less that the degree of the denominator. Of course not all rational expressions will fit into this form and so we need to take a look at a couple of examples where this isn’t the case. Example 5 Evaluate the following integral.


Math 140


4 3 2

3 2

5 6 183

x x x dxx x

− + −−

⌠⌡

Solution So, in this case the degree of the numerator is 4 and the degree of the denominator is 3. Therefore, partial fractions can’t be done on this rational expression. To fix this up we’ll need to do long division on this to get it into a form that we can deal with. Here is the work for that.

( )

( )

3 2 4 3 2

4 3

3 2

3 2

2

3 5 6 18

3

2 6 18

2 6

18

x

x x x x x

x x

x x

x x

−

− − + −

− −

− + −

− − +

−

So, from the long division we see that,

4 3 2

3 2 3 2

5 6 18 1823 3

x x x xx x x x

− + −= − −

− −

and the integral becomes,

4 3 2

3 2 3 2

3 2

5 6 18 1823 3

1823

x x x dx x dxx x x x

x dx dxx x

− + −= − −

− −

= − −−

⌠ ⌠ ⌡⌡

⌠⌡∫

The first integral we can do easily enough and the second integral is now in a form that allows us to do partial fractions. So, let’s get the general form of the partial fractions for the second integrand.

( )2 2

183 3

A B Cx x x x x

= + +− −

Setting numerators equal gives us, ( ) ( ) 218 3 3Ax x B x Cx= − + − + Now, there is a variation of the method we used in the first couple of examples that will work here. There are a couple of values of x that will allow us to quickly get two of the three constants, but there is no value of x that will just hand us the third. What we’ll do in this example is pick x’s to get the two constants that we can easily get and then we’ll just pick another value of x that will be easy to work with (i.e. it won’t give large/messy numbers anywhere) and then we’ll use the fact that we also know the other two constants to find


Math 140


the third.

( )( )( ) ( )

0 18 3 6

3 18 9 2

1 18 2 2 2 14 2

x B B

x C C

x A B C A A

= = − ⇒ = −

= = ⇒ =

= = − + − + = − + ⇒ = −


4 3 2

3 2 2

2

5 6 18 2 6 223 3

1 62 2ln 2ln 32

x x x dx x dx dxx x x x x

x x x x cx

− + −= − − − − +

− −

= − + − − − +

⌠ ⌠ ⌡⌡ ∫

In the previous example there were actually two different ways of dealing with the x2 in the denominator. One is to treat is as a quadratic which would give the following term in the decomposition

2

Ax Bx+

and the other is to treat it as a linear term in the following way,

( )22 0x x= − which gives the following two terms in the decomposition,

2

A Bx x

+

We used the second way of thinking about it in our example. Notice however that the two will give identical partial fraction decompositions. So, why talk about this? Simple. This will work for x2, but what about x3 or x4? In these cases we really will need to use the second way of thinking about these kinds of terms.

3 42 3 2 3 4

A B C A B C Dx xx x x x x x x

⇒ + + ⇒ + + +

Let’s take a look at one more example. Example 6 Evaluate the following integral.

2

2 1x dx

x −⌠⌡

Solution In this case the numerator and denominator have the same degree. As with the last example we’ll need to do long division to get this into the correct form. I’ll leave the details of that to you to check.

2

2 2 2

1 111 1 1

x dx dx dx dxx x x

= + = +− − −

⌠ ⌠ ⌠ ⌡ ⌡⌡ ∫


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So, we’ll need to partial fraction the second integral. Here’s the decomposition.

( )( )

11 1 1 1

A Bx x x x

= +− + − +

Setting numerator equal gives, ( ) ( )1 1 1A x B x= + + − Picking value of x gives us the following coefficients.

( )

( )

11 1 22

11 1 22

x B B

x A A

= − = − ⇒ = −

= = ⇒ =


2 1 12 2

2 1 1 11 1ln 1 ln 12 2

x dx dx dxx x x

x x x c

= + −− − +

= + − − + +

⌠ ⌠ ⌡⌡ ∫


Math 140


Integrals Involving Roots In this section we’re going to look at an integration technique that can be useful for some integrals with roots in them. We’ve already seen some integrals with roots in them. Some can be done quickly with a simple Calculus I substitution and some can be done with trig substitutions. However, not all integrals with roots will allow us to use one of these methods. Let’s look at a couple of examples to see another technique that can be used on occasion to help with these integrals. Example 1 Evaluate the following integral.

3

23

x dxx+−

⌠⌡

Solution Sometimes when faced with an integral that contains a root we can use the following substitution to simplify the integral into a form that can be easily worked with.

3 3u x= − So, instead of letting u be the stuff under the radical as we often did in Calculus I we let u be the whole radical. Now, there will be a little more work here since we will also need to know what x is so we can substitute in for that in the numerator and so we can compute the differential, dx. This is easy enough to get however. Just solve the substitution for x as follows, 3 23 3x u dx u du= + = Using this substitution the integral is now,

( )

( ) ( )

32 4

5 2

5 23 3

3 23 3 15

3 155 23 153 35 2

uu du u u du

u

u u c

x x c

+ += +

= + +

= − + − +

⌠⌡ ∫

So, sometimes, when an integral contains the root ( )n g x the substitution,

( )nu g x= can be used to simplify the integral into a form that we can deal with. Let’s take a look at another example real quick. Example 2 Evaluate the following integral.


Math 140


23 10

dxx x− +

⌠⌡

Solution We’ll do the same thing we did in the previous example. Here’s the substitution and the extra work we’ll need to do to get x in terms of u.

210 10 2u x x u dx u du= + = − = With this substitution the integral is,

( )2 2

2 2 4210 3 3 103 10

udx u du duu u u ux x

= =− − − −− +

⌠ ⌠ ⌠ ⌡ ⌡⌡

This integral can now be done with partial fractions.

( )( )

45 2 5 2

u A Bu u u u

= +− + − +

Setting numerators equal gives, ( ) ( )4 2 5u A u B u= + + − Picking value of u gives the coefficients.

( )

( )

82 8 77205 20 77

u B B

u A A

= − − = − =

= = =


20 87 72

5 23 1020 8ln 5 ln 27 720 8ln 10 5 ln 10 27 7

dx duu ux x

u u c

x x c

= +− +− +

= − + + +

= + − + + + +

⌠ ⌠ ⌡⌡

So, we’ve seen a nice method to eliminate roots from the integral and put into a form that we can deal with. Note however, that this won’t always work and sometimes the new integral will be just as difficult to do.


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Integrals Involving Quadratics To this point we’ve seen quite a few integrals that involve quadratics. A couple of examples are,

2 12 2 2

1 1 1ln tan2

x xdx x a c dxx a x a a a

− = ± + = ± + ⌠⌠ ⌡ ⌡

We also saw that integrals involving 2 2 2b x a− , 2 2 2a b x− and 2 2 2a b x+ could be done with a trig substitution. Notice however that all of these integrals were missing an x term. They all consist of a quadratic term and a constant. Some integrals involving general quadratics are easy enough to do. For instance, the following integral can be done with a quick substitution.

( )( )2

2

2

2 3 1 1 4 12 1 4 2 34 12 1 4

1 ln 4 12 14

x dx du u x x du x dxx x u

x x c

+= = + − = +

+ −

= + − +

⌠ ⌠ ⌡ ⌡

Some integrals with quadratics can be done with partial fractions. For instance,

2

10 6 4 2 24ln 5 ln 3 13 16 5 5 3 1 3

x dx dx x x cx x x x

−= − = + − + +

+ + + +⌠ ⌠ ⌡ ⌡

Unfortunately, these methods won’t work on a lot of integrals. A simple substitution will only work if the numerator is a constant multiple of the derivative of the denominator and partial fractions will only work if the denominator can be factored. This section is how to deal with integrals involving quadratics when the techniques that we’ve looked at to this point simply won’t work. Back in the Trig Substitution section we saw how to deal with square roots that had a general quadratic in them. Let’s take a quick look at another one like that since the idea involved in doing that kind of integral is exactly what we are going to need for the other integrals in this section. Example 1 Evaluate the following integral.

2 4 5x x dx+ +∫ Solution Recall from the Trig Substitution section that in order to do a trig substitution here we first needed to complete the square on the quadratic. This gives,

( )22 24 5 4 4 4 5 2 1x x x x x+ + = + + − + = + +


Math 140


After completing the square the integral becomes,

( )22 4 5 2 1x x dx x dx+ + = + +∫ ∫ Upon doing this we can identify the trig substitution that we need. Here it is, 22 tan tan 2 secx x dx dθ θ θ θ+ = = − =

( )2 2 22 1 tan 1 sec sec secx θ θ θ θ+ + = + = = = Recall that since we are doing an indefinite integral we can drop the absolute value bars. Using this substitution the integral becomes,

( )

2 34 5 sec

1 sec tan ln sec tan2

x x dx d

c

θ θ

θ θ θ θ

+ + =

= + + +

∫ ∫

We can finish the integral out with the following right triangle.

2

22 4 5tan sec 4 51 1

x x x x xθ θ+ + +

= = = + +

( )( )2 2 214 5 2 4 5 ln 2 4 52

x x dx x x x x x x c+ + = + + + + + + + + +∫

So, by completing the square we were able to take an integral that had a general quadratic in it and convert it into a form that allowed use a known integration technique. Let’s do a quick review of completing the square before proceeding. Here is the general completing the square formula that we’ll use.

2 2 2 2

2 2

2 2 2 4b b b bx bx c x bx c x c + + = + + − + = + + −

This will always take a general quadratic and write it in terms of a squared term and a constant term. Recall as well that in order to do this we must have a coefficient of one in front of the x2. If not we’ll need to factor out the coefficient before completing the square. In other words,


Math 140


2 2

complete the square on this!

b cax bx c a x xa a

+ + = + + 14243

Now, let’s see how completing the square can be used to do integrals that we aren’t able to do at this point. Example 2 Evaluate the following integral.

2

12 3 2

dxx x− +

⌠⌡

Solution Okay, this doesn’t factor so partial fractions just won’t work on this. Likewise, since the numerator is just “1” we can’t use the substitution 22 3 8u x x= − + . So, let’s see what happens if we complete the square on the denominator.

2 2

2

2

32 3 2 2 123 9 92 12 16 16

3 724 16

x x x x

x x

x

− + = − + = − + − +

− +

With this the integral is,

( )22 3 7

4 16

1 1 12 3 2 2

dx dxx x x

=− + − +

⌠⌠ ⌡ ⌡

Now this may not seem like all that great of a change. However, notice that we can now use the following substitution.

34

u x du dx= − =

and the integral is now,

2 2 716

1 1 12 3 2 2

dx dux x u

=− + +

⌠⌠ ⌡ ⌡

We can now see that this is an inverse tangent! So, using the formula from above we get,

12

1

1 1 4 4tan2 3 2 2 7 7

2 4 3tan7 7

udx cx x

x c

−

−

= + − + − = +

⌠⌡


Math 140



2

3 110 28x dx

x x−

+ +⌠⌡

Solution This example is a little different from the previous one. In this case we do have an x in the numerator however the numerator still isn’t a multiple of the derivative of the denominator and so a simple Calculus I substitution won’t work. So, let’s again complete the square on the denominator and see what we get,

( )22 210 28 10 25 25 28 5 3x x x x x+ + = + + − + = + + Upon completing the square the integral becomes,

( )22

3 1 3 110 28 5 3x xdx dx

x x x− −

=+ + + +

⌠⌠ ⌡ ⌡

At this point we can use the same type of substitution that we did in the previous example. The only real difference is that we’ll need to make sure that we plug the substitution back into the numerator as well. 5 5u x x u dx du= + = − =

( )

( )

2 2

2 2

2 1

2 1

3 5 13 110 28 3

3 163 3

3 16ln 3 tan2 3 33 16 5ln 5 3 tan2 3 3

ux dx dux x u

u duu u

uu c

xx c

−

−

− −−=

+ + +

= −+ +

= + − +

+ = + + − +

⌠⌠ ⌡ ⌡

⌠⌡

So, in general when dealing with an integral in the form,

2

Ax B dxax bx c

++ +

⌠⌡

(1)

Here we are going to assume that the denominator doesn’t factor and the numerator isn’t a constant multiple of the derivative of the denominator. In these cases we complete the square on the denominator and then do a substitution that will yield an inverse tangent and/or a logarithm depending on the exact form of the numerator.


Math 140


Let’s now take a look at a couple of integrals that are in the same general form as (1) except the denominator will also be raised to a power. In other words, let’s look at integrals in the form,

( )2 n

Ax B dxax bx c

+

+ +

⌠⌡

(2)


( )32 6 11

x dxx x− +

⌠⌡

Solution For the most part this integral will work the same as the previous two with one exception that will occur down the road. So, let’s start by completing the square on the quadratic in the denominator.

( )22 26 11 6 9 9 11 3 2x x x x x− + = − + − + = − + The integral is then,

( ) ( )

3 322 6 11 3 2

x xdx dxx x x

= − + − +

⌠⌠ ⌡ ⌡

Now, we will use the same substitution that we’ve used to this point in the previous two examples. 3 3u x x u dx du= − = + =

( ) ( )

( ) ( )

3 32 2

3 32 2

3

6 11 2

3

2 2

x udx dux x u

u du duu u

+=

− + +

= ++ +

⌠ ⌠ ⌡ ⌡

⌠ ⌠ ⌡ ⌡

Now, here is where the differences start cropping up. The first integral can be done with the substitution 2 2v u= + and isn’t too difficult. The second integral however, can’t be done with the substitution used on the first integral and it isn’t an inverse tangent. It turns out that a trig substitution will work nicely on the second integral and it will be the same as we did when we had square roots in the problem.

22 tan 2 secu du dθ θ θ= = With these two substitutions the integrals become,


Math 140


( ) ( )( )

( )

( ) ( )

( )( )

( )( )

23 332 2

2

32 2

2

2 32 2

2 42

422

1 1 3 2 sec26 11 2 tan 2

1 1 3 2 sec4 8 tan 1

1 1 3 2 sec4 82 sec

1 1 3 2 14 8 sec3 2

1 1 3 2 cos4 83 2

x dx dv dvx x

dv

du

dx

dx

θ θθ

θθ

θ

θθ

θ

θθ

θ θ

= +− + +

= − ++

= − ++

= − +− +

= − +− +

⌠ ⌠⌠ ⌡⌡ ⌡

⌠⌡

⌠⌡

⌠⌡

∫

Okay, at this point we’ve got two options for the remaining integral. We can either use the ideas we learned in the section about integrals involving trig integrals or we could use the following formula.

1 21 1cos sin cos cosm m mmd dm m

θ θ θ θ θ θ− −−= +∫ ∫

Let’s use this formula to do the integral.

4 3 2

3 0 0

3

1 3cos sin cos cos4 41 3 1 1sin cos sin cos cos cos 1!4 4 2 21 3 3sin cos sin cos4 8 8

d d

d

θ θ θ θ θ θ

θ θ θ θ θ θ θ

θ θ θ θ θ

= +

= + + =

= + +

∫ ∫

∫

Next, let’s use the following right triangle to get this back to x’s.

( ) ( )2 2

3 3 2tan sin cos2 2 3 2 3 2

u x x

x xθ θ θ

− −= = = =

− + − +


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The cosine integral is then,

( )( )( )

( )( )

( )( ) ( )

4 12 22

12 22

2 2 3 2 31 3 3 3cos tan4 8 8 23 23 2

2 3 3 2 3 3 3tan2 8 8 23 23 2

x x xdxx

x x xxx

θ θ −

−

− − − = + + − + − +

− − − = + + − + − +

∫

All told then the original integral is,

( ) ( )( )

( )( ) ( )

( )( ) ( )

3 22 2

12 22

12 22

1 146 11 3 2

3 2 2 3 3 2 3 3 3tan8 2 8 8 23 23 2

1 3 11 9 3 9 2 3tan8 32 64 23 23 2

x dxx x x

x x xxx

x x x cxx

−

−

= − +− + − +

− − − + + − + − +

− − − = + + + − + − +

⌠⌡

It’s a long and messy answer, but there it is. Example 5 Evaluate the following integral.

( )22

3

4 2

x dxx x

−

− −

⌠⌡

Solution As with the other problems we’ll first complete the square on the denominator.

( ) ( ) ( )( ) ( )2 22 2 24 2 2 4 2 1 1 4 1 5 5 1x x x x x x x x− − = − + − = − + + − − = − + − = − +

The integral is,

( ) ( )

2 222

3 3

4 2 5 1

x xdx dxx x x

− −=

− − − +

⌠⌠ ⌡ ⌡

Now, let’s do the substitution. 1 1u x x u dx du= + = − = and the integral is now,

( ) ( )

( ) ( )

2 22 2

2 22 2

3 4

4 2 5

4

5 5

x udx dux x u

u du duu u

− −=

− − −

= −− −

⌠ ⌠ ⌡ ⌡

⌠ ⌠ ⌡ ⌡


Math 140


In the first integral we’ll use the substitution 25v u= − and in the second integral we’ll use the following trig substitution

5 sin 5 cosu du dθ θ θ= = Using these substitutions the integral becomes,

( ) ( )( )

( )

( )

2 222 2

22

4

3

3 1 1 4 5 cos24 2 5 5sin

1 1 4 5 cos2 25 1 sin

1 1 4 5 cos2 25 cos1 1 4 5 sec2 251 1 2 5 sec tan ln sec tan2 25

x dx dv dvx x

dv

dv

dv

cv

θ θθ

θθ

θ

θθ

θ

θ θ

θ θ θ θ

−= − −

− − −

= −−

= −

= −

= − + + +

⌠ ⌠⌠ ⌡⌡ ⌡

⌠⌡

⌠⌡

∫

We’ll need the following right triangle to finish this integral out.

( ) ( )2 2

1 5 1sin sec tan5 5 5 1 5 1

u x x

x xθ θ θ

+ += = = =

− + − +

So, going back to x’s the integral becomes,

( )

( )( ) ( ) ( )

( ) ( )

2 22 2 22

2 2

5 13 1 1 2 5 5 1ln2 5 25 5 14 2 5 1 5 1

1 4 1 2 5 1 5ln10 255 1 5 1

xx xdx cu xx x x x

x x cx x

+− + = − + + + − − + − − − + − +

− + += + +

− + − +

⌠⌡


Math 140


Often the following formula is needed when using the trig substitution that we used in the previous example.

2 21 2sec tan sec sec1 1

m m mmd dm m

θ θ θ θ θ θ− −−= +

− −∫ ∫

Note that we’ll only need the two trig substitutions that we used here. The third trig substitution that we used will not be needed here.


Math 140


Using Integral Tables Note : Of all the notes that I’ve written up for download, this is the one section that is tied to the book that we are currently using here at Lamar University. In this section we discuss using tables of integrals to help us with some integrals. However, I haven’t had the time to construct a table of my own and so I will be using the tables given in Stewart’s Calculus, Early Transcendentals (6th edition). As soon as I get around to writing my own table I’ll post it online and make any appropriate changes to this section. So, with that out of the way let’s get on with this section. This section is entitled Using Integral Tables and we will be using integral tables. However, at some level, this isn’t really the point of this section. To a certain extent the real subject of this section is how to take advantage of known integrals to do integrals that may not look like anything the ones that we do know how to do or are given in a table of integrals. For the most part we’ll be doing this by using substitution to put integrals into a form that we can deal with. However, not all of the integrals will require a substitution. For some integrals all that we need to do is a little rewriting of the integrand to get into a form that we can deal with. We’ve already related a new integral to one we could deal with least once. In the last example in the Trig Substitution section we looked at the following integral.

4 21x x dx+∫e e At first glance this looks nothing like a trig substitution problem. However, with the substitution

xu = e we could turn the integral into,

3 21u u du+∫ which definitely is a trig substitution problem ( tanu θ= ). We actually did this process in a single step by using tanx θ=e , but the point is that with a substitution we were able to convert an integral into a form that we could deal with. So, let’s work a couple examples using substitutions and tables. Example 1 Evaluate the following integral.

2

2

7 9x dxx+⌠

⌡

Solution So, the first thing we should do is go to the tables and see if there is anything in the tables that is close to this. In the tables in Stewart we find the following integral,


Math 140


( )2 2 2 2

2 22 lna u a udu u a u c

u u+ +

= − + + + +⌠⌡

This is nearly what we’ve got in our integral. The only real difference is that we’ve got a coefficient in front of the x2 and the formula doesn’t. This is easily enough dealt with. All we need to do is the following manipulation on the integrand.

( )27 2 22 7 7

9 9 92 2 2 2

9 37 9 3x x xx dx dx dx dx

x x x x

+ + ++= = =

⌠ ⌠ ⌠⌠ ⌡ ⌡ ⌡⌡

So, we can now use the formula with 7

3a = .

22 7

9 22

7 9 73 ln9

xx dx x x cx x

++ = − + + + +

⌠⌡


cossin 9sin 4

x dxx x −

⌠⌡

Solution Going through our tables we aren’t going to find anything that looks like this in them. However, notice that with the substitution sinu x= we can rewrite the integral as,

cos 1sin 9sin 4 9 4

x dx dux x u u

=− −

⌠ ⌠ ⌡ ⌡

and this is in the tables.

1

1 1 ln if 0

2 tan if 0

a bu adu c au a bu a a bu a

a bu c aaa

−

+ −= + >

+ + +

+= + < −−

⌠⌡

Notice that this is a formula that will depend upon the value of a. This will happen on occasion. In our case we have 4a = − and 9b = so we’ll use the second formula.

( ) ( )

1

1

cos 2 9 4tan4sin 9sin 4 4

9sin 4tan2

x udx cx x

x c

−

−

−= + − −− − −

−= +

⌠⌡

This final example uses a type of formula known as a reduction formula.


Math 140



4cot2x dx

⌠⌡

Solution

We’ll first need to use the substitution 2xu = since none of the formulas in our tables have that in

them. Doing this gives,

4 4cot 2 cot2x dx u du =

⌠⌡ ∫

To help us with this integral we’ll use the following formula.

1 21cot cot cot1

n n nu du u u dun

− −−= −

−∫ ∫

Formulas like this are called reduction formulas. Reduction formulas generally don’t explicitly give the integral. Instead they reduce the integral to an easier one. In fact they often reduce the integral to a different version of itself! For our integral we’ll use 4n = .

4 3 21cot 2 cot cot2 3x dx u u du = − −

⌠⌡ ∫

At this stage we can either reuse the reduction formula with 2n = or use the formula

2cot cotu du u u c= − − +∫ We’ll reuse the reduction formula with 2n = so we can address something that happens on occasion.


Math 140


4 3 0 0

3

3

3

1 1cot 2 cot cot cot cot 1!2 3 1

2 cot 2cot 232 cot 2cot 232 cot 2cot3 2 2

x dx u u u du u

u u du

u u u c

x x x c

= − − − − =

= − + +

= − + + +

= − + + +

⌠⌡ ∫

∫

Don’t forget that 0 1a = . Often people forget that and then get stuck on the final integral! There really wasn’t a lot to this section. Just don’t forget that sometimes a simple substitution or rewrite of an integral can take it from undoable to doable.


Math 140


Integration Strategy We’ve now seen a fair number of different integration techniques and so we should probably pause at this point and talk a little bit about a strategy to use for determining the correct technique to use when faced with an integral. There are a couple of points that need to be made about this strategy. First, it isn’t a hard and fast set of rules for determining the method that should be used. It is really nothing more than a general set of guidelines that will help us to identify techniques that may work. Some integrals can be done in more than one way and so depending on the path you take through the strategy you may end up with a different technique than somebody else who also went through this strategy. Second, while the strategy is presented as a way to identify the technique that could be used on an integral also keep in mind that, for many integrals, it can also automatically exclude certain techniques as well. When going through the strategy keep two lists in mind. The first list is integration techniques that simply won’t work and the second list is techniques that look like they might work. After going through the strategy and the second list has only one entry then that is the technique to use. If, on the other hand, there are more than one possible technique to use we will then have to decide on which is liable to be the best for us to use. Unfortunately there is no way to teach which technique is the best as that usually depends upon the person and which technique they find to be the easiest. Third, don’t forget that many integrals can be evaluated in multiple ways and so more than one technique may be used on it. This has already been mentioned in each of the previous points, but is important enough to warrant a separate mention. Sometimes one technique will be significantly easier than the others and so don’t just stop at the first technique that appears to work. Always identify all possible techniques and then go back and determine which you feel will be the easiest for you to use. Next, it’s entirely possible that you will need to use more than one method to completely do an integral. For instance a substitution may lead to using integration by parts or partial fractions integral. Finally, in my class I will accept any valid integration technique as a solution. As already noted there is often more than one way to do an integral and just because I find one technique to be the easiest doesn’t mean that you will as well. So, in my class, there is no one right way of doing an integral. You may use any integration technique that I’ve taught you in this class or you learned in Calculus I to evaluate integrals in this class. In other words, always take the approach that you find to be the easiest. Note that this final point is more geared towards my class and it’s completely possible that your instructor may not agree with this and so be careful in applying this point if you aren’t in my class.


Math 140


Okay, let’s get on with the strategy.

1. Simplify the integrand, if possible. This step is very important in the integration process. Many integrals can be taken from impossible or very difficult to very easy with a little simplification or manipulation. Don’t forget basic trig and algebraic identities as these can often be used to simplify the integral. We used this idea when we were looking at integrals involving trig functions. For example consider the following integral.

2cos x dx∫

This integral can’t be done as is however, simply by recalling the identity,

( )( )2 1cos 1 cos 22

x x= +

the integral becomes very easy to do. Note that this example also shows that simplification does not necessarily mean that we’ll write the integrand in a “simpler” form. It only means that we’ll write the integrand into a form that we can deal with and this is often longer and/or “messier” than the original integral.

2. See if a “simple” substitution will work. Look to see if a simple substitution can be used instead of the often more complicated methods from Calculus II. For example consider both if the following integrals.

22 1

1x dx x x dx

x−

−⌠⌡ ∫

The first integral can be done with partial fractions and the second could be done with a trig substitution. However, both could also be evaluated using the substitution 2 1u x= − and the work involved in the substitution would be significantly less than the work involved in either partial fractions or trig substitution. So, always look for quick, simple substitutions before moving on to the more complicated Calculus II techniques.

3. Identify the type of integral. Note that any integral may fall into more than one of these types. Because of this fact it’s usually best to go all the way through the list and identify


Math 140


all possible types since one may be easier than the other and it’s entirely possible that the easier type is listed lower in the list.

a. Is the integrand a rational expression (i.e is the integrand a polynomial divided by a polynomial)? If so, then partial fractions may work on the integral.

b. Is the integrand a polynomial times a trig function, exponential, or logarithm? If so, then integration by parts may work.

c. Is the integrand a product of sines and cosines, secant and tangents, or cosecants and cotangents? If so, then the topics from the second section may work. Likewise, don’t forget that some quotients involving these functions can also be done using these techniques.

d. Does the integrand involve 2 2 2b x a+ , 2 2 2b x a− , or 2 2 2a b x− ? If so, then a trig substitution might work nicely.

e. Does the integrand have roots other than those listed above in it? If so, then the substitution ( )nu g x= might work.

f. Does the integrand have a quadratic in it? If so, then completing the square on the quadratic might put it into a form that we can deal with.

4. Can we relate the integral to an integral we already know how to do? In other words, can we use a substitution or manipulation to write the integrand into a form that does fit into the forms we’ve looked at previously in this chapter. A typical example here is the following integral.

2cos 1 sinx x dx+∫

This integral doesn’t obviously fit into any of the forms we looked at in this chapter. However, with the substitution sinu x= we can reduce the integral to the form,

21 u du+∫

which is a trig substitution problem.

5. Do we need to use multiple techniques? In this step we need to ask ourselves if it is possible that we’ll need to use multiple techniques. The example in the previous part is a good example. Using a substitution didn’t allow us to actually do the integral. All it did was put the integral and put it into a form that we could use a different technique on. Don’t ever get locked into the idea that an integral will only require one step to completely evaluate it. Many will require more than one step.

6. Try again. If everything that you’ve tried to this point doesn’t work then go back through the process and try again. This time try a technique that that you didn’t use the first time around.

As noted above this strategy is not a hard and fast set of rules. It is only intended to guide you through the process of best determining how to do any given integral. Note as well that the only place Calculus II actually arises is in the third step. Steps 1, 2 and 4 involve nothing more than


Math 140


manipulation of the integrand either through direct manipulation of the integrand or by using a substitution. The last two steps are simply ideas to think about in going through this strategy. Many students go through this process and concentrate almost exclusively on Step 3 (after all this is Calculus II, so it’s easy to see why they might do that….) to the exclusion of the other steps. One very large consequence of that exclusion is that often a simple manipulation or substitution is overlooked that could make the integral very easy to do. Before moving on to the next section we should work a couple of quick problems illustrating a couple of not so obvious simplifications/manipulations and a not so obvious substitution. Example 1 Evaluate the following integral.

4

tansec

x dxx

⌠⌡

Solution This integral almost falls into the form given in 3c. It is a quotient of tangent and secant and we know that sometimes we can use the same methods for products of tangents and secants on quotients. The process from that section tells us that if we have even powers of secant to strip two of them off and convert the rest to tangents. That won’t work here. We can split two secants off, but they would be in the denominator and they won’t do us any good there. Remember that the point of splitting them off is so they would be there for the substitution tanu x= . That requires them to be in the numerator. So, that won’t work and so we’ll have to find another solution method. There are in fact two solution methods to this integral depending on how you want to go about it. We’ll take a look at both. Solution 1 In this solution method we could just convert everything to sines and cosines and see if that gives us an integral we can deal with.

44

3

3

4

tan sin cossec cos

sin cos cos

1 cos4

x xdx x dxx x

x x dx u x

u du

x c

=

= =

= −

= − +

⌠ ⌠ ⌡ ⌡

∫∫

Note that just converting to sines and cosines won’t always work and if it does it won’t always work this nicely. Often there will be a lot more work that would need to be done to complete the integral.


Math 140


Solution 2 This solution method goes back to dealing with secants and tangents. Let’s notice that if we had a secant in the numerator we could just use secu x= as a substitution and it would be a fairly quick and simple substitution to use. We don’t have a secant in the numerator. However we could very easily get a secant in the numerator simply by multiplying the numerator and denominator by secant.

4 5

5

4

4

tan tan sec secsec sec

1

1 14 sec1 cos4

x x xdx dx u xx x

duu

cx

x c

= =

=

= − +

= − +

⌠ ⌠ ⌡ ⌡

⌠⌡

In the previous example we saw two “simplifications” that allowed us to do the integral. The first was using identities to rewrite the integral into terms we could deal with and the second involved multiplying the numerator and the denominator by something to again put the integral into terms we could deal with. Using identities to rewrite an integral is an important “simplification” and we should not forget about it. Integrals can often be greatly simplified or at least put into a form that can be dealt with by using an identity. The second “simplification” is not used as often, but does show up on occasion so again, it’s best to not forget about it. In fact, let’s take another look at an example in which multiplying the numerator and denominator by something will allow us to do an integral. Example 2 Evaluate the following integral.

11 sin

dxx+

⌠⌡

Solution This is an integral in which if we just concentrate on the third step we won’t get anywhere. This integral doesn’t appear to be any of the kinds of integrals that we worked in this chapter. We can do the integral however, if we do the following,

2

1 1 1 sin1 sin 1 sin 1 sin

1 sin1 sin

xdx dxx x x

x dxx

−=

+ + −−

=−

⌠ ⌠ ⌡ ⌡

⌠⌡


Math 140


This does not appear to have done anything for us. However, if we now remember the first “simplification” we looked at above we will notice that we can use an identity to rewrite the denominator. Once we do that we can further reduce the integral into something we can deal with.

2

2

2

1 1 sin1 sin cos

1 sin 1cos cos cos

sec tan sec

tan sec

xdx dxx x

x dxx x x

x x x dx

x x c

−=

+

= −

= −

= − +

⌠ ⌠ ⌡ ⌡

⌠⌡

∫

So, we’ve seen once again that multiplying the numerator and denominator by something can put the integral into a form that we can integrate. Notice as well that this example also showed that “simplifications” do not necessarily put an integral into a simpler form. They only put the integral into a form that is easier to integrate. Let’s now take a quick look at an example of a substitution that is not so obvious. Example 3 Evaluate the following integral. ( )cos x dx∫ Solution We introduced this example saying that the substitution was not so obvious. However, this is really an integral that falls into the form given by 3e in our strategy above. However, many people miss that form and so don’t think about it. So, let’s try the following substitution.

2 2u x x u dx u du= = = With this substitution the integral becomes,

( )cos 2 cosx dx u u du=∫ ∫ This is now an integration by parts integral. Remember that often we will need to use more than one technique to completely do the integral. This is a fairly simple integration by parts problem so I’ll leave the remainder of the details to you to check.

( ) ( ) ( )( )cos 2 cos sinx dx x x x c= + +∫

Before leaving this section we should also point out that there are integrals out there in the world that just can’t be done in terms of functions that we know. Some examples of these are.

( ) ( ) ( )22 sincos cos xx x

dx x dx dx dxx

− ⌠⌡∫ ∫ ∫e e


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That doesn’t mean that these integrals can’t be done at some level. If you go to a computer algebra system such as Maple or Mathematica and have it do these integrals here is what it will return the following.

( )

( )

( ) ( )

( ) ( )

2

2erf

22cos FresnelC

2

sinSi

cos Cix x

x dx x

x dx x

xdx x

xdx

π

ππ

− =

=

=

=

⌠⌡

∫

∫

∫

e

e e

So it appears that these integrals can in fact be done. However this is a little misleading. Here are the definitions of each of the functions given above. Error Function

( )0

22erfx tx dt

π−= ∫ e

The Sine Integral

( )0

sinSix tx dt

t= ⌠

⌡

The Fresnel Cosine Integral

( ) 2

0

FresnelC cos2

x

x t dtπ =

⌠⌡

The Cosine Integral

( ) ( )0

cos 1Ci lnx tx x dt

tγ

−= + + ⌠

⌡

Where γ is the Euler-Mascheroni constant. Note that the first three are simply defined in terms of themselves and so when we say we can integrate them all we are really doing is renaming the integral. The fourth one is a little different and yet it is still defined in terms of an integral that can’t be done in practice. It will be possible to integrate every integral given in this class, but it is important to note that there are integrals that just can’t be done. We should also note that after we look at Series we will be able to write down series representations of each of the integrals above.


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Improper Integrals In this section we need to take a look at a couple of different kinds of integrals. Both of these are examples of integrals that are called Improper Integrals. Let’s start with the first kind of improper integrals that we’re going to take a look at. Infinite Interval In this kind of integrals we are going to take a look at integrals that in which one or both of the limits of integration are infinity. In these cases the interval of integration is said to be over an infinite interval. Let’s take a look at an example that will also show us how we are going to deal with these integrals. Example 1 Evaluate the following integral.

21

1 dxx

∞⌠⌡

Solution This is an innocent enough looking integral. However, because infinity is not a real number we can’t just integrate as normal and then “plug in” the infinity to get an answer. To see how we’re going to do this integral let’s think of this as an area problem. So instead of

asking what the integral is, let’s instead ask what the area under ( ) 2

1f xx

= on the interval

[ )1,∞ is.

We still aren’t able to do this, however, let’s step back a little and instead ask what the area under

( )f x is on the interval [ ]1, t were 1t > and t is finite. This is a problem that we can do.

21 1

1 1 11tt

tA dxx x t

= = − = −⌠⌡

Now, we can get the area under ( )f x on [ )1,∞ simply by taking the limit of At as t goes to

infinity.

1lim lim 1 1tt tA A

t→∞ →∞

= = − =

This is then how we will do the integral itself.


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2 21 1

1

1 1lim

1lim

1lim 1 1

t

t

t

t

t

dx dxx x

x

t

∞

→∞

→∞

→∞

=

= −

= − =

⌠ ⌠ ⌡ ⌡

So, this is how we will deal with these kinds of integrals in general. We will replace the infinity with a variable (usually t), do the integral and then take the limit of the result as t goes to infinity. On a side note, notice that the area under a curve on an infinite interval was not infinity as we might have suspected it to be. In fact, it was a surprisingly small number. Of course this won’t always be the case, but it is important enough to point out that not all areas on an infinite interval will yield infinite areas. Let’s now get some definitions out of the way. We will call these integrals convergent if the associated limit exists and is a finite number (i.e. it’s not plus or minus infinity) and divergent if the associated limits either doesn’t exist or is (plus or minus) infinity. Let’s now formalize up the method for dealing with infinite intervals. There are essentially three cases that we’ll need to look at.

1. If ( )t

af x dx∫ exists for every t a> then,

( ) ( )limt

a atf x dx f x dx

→∞

∞=∫ ∫

provided the limit exists and is finite.

2. If ( )b

tf x dx∫ exists for every t b< then,

( ) ( )limb b

ttf x dx f x dx

− →−∞∞=∫ ∫

provided the limits exists and is finite.

3. If ( )c

f x dx−∞∫ and ( )

cf x dx

∞

∫ are both convergent then,

( ) ( ) ( )c

cf x dx f x dx f x dx

− −

∞ ∞

∞ ∞= +∫ ∫ ∫

Where c is any number. Note as well that this requires BOTH of the integrals to be convergent in order for this integral to also be convergent. If either of the two integrals is divergent then so is this integral.

Let’s take a look at a couple more examples.


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Example 2 Determine if the follow integral is convergent or divergent and if it’s convergent find its value.

1

1 dxx

∞⌠⌡

Solution So, the first thing we do is convert the integral to a limit.

1 1

1 1limt

tdx dx

x x

∞

→∞=⌠ ⌠

⌡ ⌡

Now, do the integral and the limit.

( )

( )( )1

1

1 lim ln

lim ln ln1

t

t

t

dx xx

t

∞

→∞

→∞

=

= −

= ∞

⌠⌡

So, the limit is infinite and so the integral is divergent. If we go back to thinking in terms of area notice that the area under ( ) 1

xg x = on the interval

[ )1,∞ is infinite. This is in contrast to the area under ( ) 21xf x = which was quite small. There

really isn’t all that much difference between these two functions and yet there is a large difference in the area under them. We can actually extend this out to the following fact. Fact If 0a > then

1p

adx

x

∞⌠⌡

is convergent if 1p > and divergent if 1p ≤ . One thing to note about this fact is that it’s in essence saying that if an integrand goes to zero fast enough then the integral will converge. How fast is fast enough? If we use this fact as a guide it looks like integrands that go to zero faster than 1

x goes to zero will probably converge. Let’s take a look at a couple more examples. Example 3 Determine if the following integral is convergent or divergent. If it is convergent find its value.

0 1

3dx

x−∞ −⌠⌡


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Solution There really isn’t much to do with these problems once you know how to do them. We’ll convert the integral to a limit/integral pair, evaluate the integral and then the limit.

( )

0 0

0

1 1lim3 3

lim 2 3

lim 2 3 2 3

2 3

tt

t t

t

dx dxx x

x

t

→−∞−∞

→−∞

→−∞

=− −

= − −

= − + −

= − + ∞= ∞

⌠ ⌠ ⌡ ⌡

So, the limit is infinite and so this integral is divergent. Example 4 Determine if the following integral is convergent or divergent. If it is convergent find its value.

2xx dx

∞ −

−∞∫ e

Solution In this case we’ve got infinities in both limits and so we’ll need to split the integral up into two separate integrals. We can split the integral up at any point, so let’s choose 0a = since this will be a convenient point for the evaluate process. The integral is then,

0

0

2 2 2x x xx dx x dx x dx∞ ∞− − −

−∞ −∞= +∫ ∫ ∫e e e

We’ve now got to look at each of the individual limits.

0 0

0

2 2

2

2

lim

1lim2

1 1lim2 2

12

tt

tt

t

t

x x

x

x dx x dx− −

−∞ →−∞

−

→−∞

−

→−∞

=

= −

= − +

= −

∫ ∫e e

e

e

So, the first integral is convergent. Note that this does NOT mean that the second integral will also be convergent. So, let’s take a look at that one.


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0 0

0

2 2

2

2

lim

1lim2

1 1lim2 2

12

t

t

t

t

t

t

x x

x

x dx x dx∞ − −

→∞

−

→∞

−

→∞

=

= −

= − +

=

∫ ∫e e

e

e

This integral is convergent and so since they are both convergent the integral we were actually asked to deal with is also convergent and its value is,

0

0

2 2 2 1 1 02 2

x x xx dx x dx x dx∞ ∞− − −

−∞ −∞= + = − + =∫ ∫ ∫e e e

Example 5 Determine if the following integral is convergent or divergent. If it is convergent find its value.

2sin x dx

∞

−∫

Solution First convert to a limit.

( )( )

2 2

2

sin lim sin

lim cos

lim cos 2 cos

t

t

t

t

t

x dx x dx

x

t

∞

− −→∞

−→∞

→∞

=

= −

= −

∫ ∫

This limit doesn’t exist and so the integral is divergent. In most examples in a Calculus II class that are worked over infinite intervals the limit either exists or is infinite. However, there are limits that don’t exist, as the previous example showed, so don’t forget about those. Discontinuous Integrand We now need to look at the second type of improper integrals that we’ll be looking at in this section. These are integrals that have discontinuous integrands. The process here is basically the same with one on subtle difference. Here are the general cases that we’ll look at for these integrals.

1. If ( )f x is continuous on the interval [ ),a b and not continuous at x b= then,

( ) ( )limb t

a at bf x dx f x dx

−→=∫ ∫

provided the limit exists and is finite. Note as well that we do need to use a left hand limit here since the interval of integration is entirely on the left side of the upper limit.


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2. If ( )f x is continuous on the interval ( ],a b and not continuous at x a= then,

( ) ( )limb b

a tt af x dx f x dx

+→=∫ ∫

provided the limit exists and is finite. In this case we need to use a right hand limit here since the interval of integration is entirely on the right side of the lower limit.

3. If ( )f x is not continuous at x c= where a c b< < and ( )c

af x dx∫ and ( )

b

cf x dx∫

are both convergent then,

( ) ( ) ( )b c b

a a cf x dx f x dx f x dx= +∫ ∫ ∫

As with the infinite interval case this requires BOTH of the integrals to be convergent in order for this integral to also be convergent. If either of the two integrals is divergent then so is this integral.

4. If ( )f x is not continuous at x a= and x b= and if ( )c

af x dx∫ and ( )

b

cf x dx∫ are

both convergent then,

( ) ( ) ( )b c b

a a cf x dx f x dx f x dx= +∫ ∫ ∫

Where c is any number. Again, this requires BOTH of the integrals to be convergent in order for this integral to also be convergent.

Note that the limits in these cases really do need to be right or left handed limits. Since we will be working inside the interval of integration we will need to make sure that we stay inside that interval. This means that we’ll use one-sided limits to make sure we stay inside the interval. Let’s do a couple of examples of these kinds of integrals. Example 6 Determine if the following integral is convergent or divergent. If it is convergent find its value.

3

0

13

dxx−

⌠⌡

Solution The problem point is the upper limit so we are in the first case above.

( )( )

3

30 0

3 0

3

1 1lim3 3

lim 2 3

lim 2 3 2 3

2 3

t

t

t

t

t

dx dxx x

x

t

−

−

−

→

→

→

=− −

= − −

= − −

=

⌠ ⌠ ⌡ ⌡


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The limit exists and is finite and so the integral converges and the integrals value is 2 3 . Example 7 Determine if the following integral is convergent or divergent. If it is convergent find its value.

3

32

1 dxx−

⌠⌡

Solution This integrand is not continuous at 0x = and so we’ll need to split the integral up at that point.

3 0 3

3 3 32 2 0

1 1 1dx dx dxx x x− −

= +⌠ ⌠ ⌠ ⌡ ⌡ ⌡

Now we need to look at each of these integrals and see if they are convergent.

0

3 302 2

202

20

1 1lim

1lim2

1 1lim2 8

t

t

t

t

t

dx dxx x

x

t

−

−

−

→− −

→−

→

=

= −

= − +

= −∞

⌠ ⌠ ⌡ ⌡

At this point we’re done. One of the integrals is divergent that means the integral that we were asked to look at is divergent. We don’t even need to bother with the second integral. Before leaving this section lets note that we can also have integrals that involve both of these cases. Consider the following integral. Example 8 Determine if the following integral is convergent or divergent. If it is convergent find its value.

20

1 dxx

∞⌠⌡

Solution This is an integral over an infinite interval that also contains a discontinuous integrand. To do this integral we’ll need to split it up into two integrals. We can split it up anywhere, but pick a value that will be convenient for evaluation purposes.

1

2 2 20 0 1

1 1 1dx dx dxx x x

∞ ∞

= +⌠ ⌠ ⌠ ⌡ ⌡ ⌡

In order for the integral in the example to be convergent we will need BOTH of these to be convergent. If one or both are divergent then the whole integral will also be divergent. We know that the second integral is convergent by the fact given in the infinite interval portion


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above. So, all we need to do is check the first integral.

1 1

2 200

1

0

0

1 1lim

1lim

1lim 1

t t

tt

t

dx dxx x

x

t

+

+

+

→

→

→

=

= −

= − +

= ∞

⌠ ⌠ ⌡ ⌡

So, the first integral is divergent and so the whole integral is divergent.


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Comparison Test for Improper Integrals Now that we’ve seen how to actually compute improper integrals we need to address one more topic about them. Often we aren’t concerned with the actual value of these integrals. Instead we might only be interested in whether the integral is convergent or divergent. Also, there will be some integrals that we simply won’t be able to integrate and yet we would still like to know if they converge or diverge. To deal with this we’ve got a test for convergence or divergence that we can use to help us answer the question of convergence for an improper integral. We will give this test only for a sub-case of the infinite interval integral, however versions of the test exist for the other sub-cases of the infinite interval integrals as well as integrals with discontinuous integrands. Comparison Test If ( ) ( ) 0f x g x≥ ≥ on the interval [ ),a ∞ then,

1. If ( )a

f x dx∞

∫ converges then so does ( )a

g x dx∞

∫ .

2. If ( )a

g x dx∞

∫ diverges then so does ( )a

f x dx∞

∫ .

Note that if you think in terms of area the Comparison Test makes a lot of sense. If ( )f x is

larger than ( )g x then the area under ( )f x must also be larger than the area under ( )g x .

So, if the area under the larger function is finite (i.e. ( )a

f x dx∞

∫ converges) then the area under

the smaller function must also be finite (i.e. ( )a

g x dx∞

∫ converges). Likewise, if the area under

the smaller function is infinite (i.e. ( )a

g x dx∞

∫ diverges) then the area under the larger function

must also be infinite (i.e. ( )a

f x dx∞

∫ diverges).

Be careful not to misuse this test. If the smaller function converges there is no reason to believe that the larger will also converge (after all infinity is larger than a finite number…) and if the larger function diverges there is no reason to believe that the smaller function will also diverge. Let’s work a couple of example using the comparison test. Note that all we’ll be able to do is determine the convergence of the integral. We won’t be able to determine the value of the integrals and so won’t even bother with that.


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Example 1 Determine if the following integral is convergent or divergent.

2

22

cos x dxx

∞⌠⌡

Solution Let’s take a second and think about how the Comparison Test works. If this integral is convergent then we’ll need to find a larger function that also converges on the same interval. Likewise, if this integral is divergent then we’ll need to find a smaller function that also diverges. So, it seems like it would be nice to have some idea as to whether the integral converges or diverges ahead of time so we will know whether we will need to look for a larger (and convergent) function or a smaller (and divergent) function. To get the guess for this function let’s notice that the numerator is nice and bounded and simply won’t get too large. Therefore, it seems likely that the denominator will determine the convergence/divergence of this integral and we know that

22

1 dxx

∞⌠⌡

converges since 2 1p = > by the fact in the previous section. So let’s guess that this integral will converge. So we now know that we need to find a function that is larger than

2

2

cos xx

and also converges. Making a fraction larger is actually a fairly simple process. We can either make the numerator larger or we can make the denominator smaller. In this case can’t do a lot about the denominator. However we can use the fact that 20 cos 1x≤ ≤ to make the numerator larger (i.e. we’ll replace the cosine with something we know to be larger, namely 1). So,

2

2 2

cos 1xx x

≤

Now, as we’ve already noted

22

1 dxx

∞⌠⌡

converges and so by the Comparison Test we know that

2

22

cos x dxx

∞⌠⌡

must also converge.


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3

1x dx

x

∞

+⌠⌡ e

Solution Let’s first take a guess about the convergence of this integral. As noted after the fact in the last section about

1p

adx

x

∞⌠⌡

if the integrand goes to zero faster than 1x

then the integral will probably converge. Now, we’ve

got an exponential in the denominator which is approaching infinity much faster than the x and so it looks like this integral should probably converge. So, we need a larger function that will also converge. In this case we can’t really make the numerator larger and so we’ll need to make the denominator smaller in order to make the function larger as a whole. We will need to be careful however. There are two ways to do this and only one, in this case only one, of them will work for us. First, notice that since the lower limit of integration is 3 we can say that 3 0x ≥ > and we know that exponentials are always positive. So, the denominator is the sum of two positive terms and if we were to drop one of them the denominator would get smaller. This would in turn make the function larger. The question then is which one to drop? Let’s first drop the exponential. Doing this gives,

1 1xx x

<+ e

This is a problem however, since

3

1 dxx

∞⌠⌡

diverges by the fact. We’ve got a larger function that is divergent. This doesn’t say anything about the smaller function. Therefore, we chose the wrong one to drop. Let’s try it again and this time let’s drop the x.

1 1 xx xx

−< =+

ee e

Also,

( )3 3

3

3

lim

lim

tx x

t

t

t

dx dx∞ − −

→∞

− −

→∞

−

=

= − +

=

∫ ∫e e

e e

e


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So, 3

x dx∞ −∫ e is convergent. Therefore, by the Comparison test

3

1x dx

x

∞

+⌠⌡ e

is also convergent. Example 3 Determine if the following integral is convergent or divergent.

3

1x dx

x

∞

−−⌠⌡ e

Solution This is very similar to the previous example with a couple of very important differences. First, notice that the exponential now goes to zero as x increases instead of growing larger as it did in the previous example (because of the negative in the exponent). Also note that the exponential is now subtracted off the x instead of added onto it. The fact that the exponential goes to zero means that this time the x in the denominator will probably dominate the term and that means that the integral probably diverges. We will therefore need to find a smaller function that also diverges. Making fractions smaller is pretty much the same as making fractions larger. In this case we’ll need to either make the numerator smaller or the denominator larger. This is where the second change will come into play. As before we know that both x and the exponential are positive. However, this time since we are subtracting the exponential from the x if we were to drop the exponential the denominator will become larger and so the fraction will become smaller. In other words,

1 1xx x− >

− e

and we know that

3

1 dxx

∞⌠⌡

diverges and so by the Comparison Test we know that

3

1x dx

x

∞

−−⌠⌡ e

must also diverge. Example 4 Determine if the following integral is convergent or divergent.


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( )4

1

1 3sin 2xdx

x

∞ +⌠⌡

Solution First notice that as with the first example, the numerator in this function is going to be bounded since the sine is never larger than 1. Therefore, since the exponent on the denominator is less than 1 we can guess that the integral will probably diverge. We will need a smaller function that also diverges. We know that ( )40 sin 2 1x≤ ≤ . In particular, this term is positive and so if we drop it from the

numerator the numerator will get smaller. This gives,

( )41 3sin 2 1xx x

+>

and

1

1 dxx

∞⌠⌡

diverges so by the Comparison Test

( )4

1

1 3sin 2xdx

x

∞ +⌠⌡

also diverges. Okay, we’ve seen a few examples of the Comparison Test now. However, most of them worked pretty much the same way. All the functions were rational and all we did for most of them was add or subtract something from the numerator or denominator to get what we want. Let’s take a look at an example that works a little differently so we don’t get too locked into these ideas. Example 5 Determine if the following integral is convergent or divergent.

1

x

dxx

∞ −⌠⌡

e

Solution Normally, the presence of just an x in the denominator would lead us to guess divergent for this integral. However, the exponential in the numerator will approach zero so fast that instead we’ll need to guess that this integral converges. To get a larger function we’ll use the fact that we know from the limits of integration that 1x > . This means that if we just replace the x in the denominator with 1 (which is always smaller than x) we will make the denominator smaller and so the function will get larger.

1

x xx

x

− −−< =

e e e


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and we can show that

1

x dx∞ −∫ e

converges. In fact, we’ve already done this for a lower limit of 3 and changing that to a 1 won’t change the convergence of the integral. Therefore, by the Comparison Test

1

x

dxx

∞ −⌠⌡

e

also converges. We should also really work an example that doesn’t involve a rational function since there is no reason to assume that we’ll always be working with rational functions. Example 6 Determine if the following integral is convergent or divergent.

1

2x dx∞ −∫ e

Solution We know that exponentials with negative exponents die down to zero very fast so it makes sense to guess that this integral will be convergent. We need a larger function, but this time we don’t have a fraction to work with so we’ll need to do something different. We’ll take advantage of the fact that x−e is a decreasing function. This means that 1 2

1 2x xx x − −> ⇒ <e e

In other words, plug in a larger number and the function gets smaller. From the limits of integration we know that 1x > and this means that if we square x we will get larger. Or, 2 provided 1x x x> > Note that we can only say this since 1x > . This won’t be true if 1x ≤ ! We can now use the fact that x−e is a decreasing function to get,

2 xx− −<e e

So, x−e is a larger function than 2x−e and we know that

1

x dx∞ −∫ e

converges so by the Comparison Test we also know that

1

2x dx∞ −∫ e

is convergent. The last two examples made use of the fact that 1x > . Let’s take a look at an example to see how do we would have to go about these if the lower limit had been smaller than 1.


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12

x

dxx

∞ −⌠⌡

e

Solution In this case we can’t just replace x with 1

2 in the denominator and get a larger function for all x in the interval of integration as we did in Example 5 above. Remember that we need a function (that is also convergent) that is always larger than the given function.. To see why we can’t just replace x with 1

2 plug in 34x = into the denominator and compare this

to what we would have if we plug in 12x = .

4 23 4 3 1 2

x xx x

− −− −= < =

e ee e

So, for x’s in the range 12 1x≤ < we won’t get a larger function as required for use in the

Comparison Test. However, this isn’t the problem it might at first appear to be. We can always write the integral as follows,

1

1 1 12 2

10.34039

x x x

x

dx dx dxx x x

dxx

∞ ∞− − −

∞ −

= +

= +

⌠ ⌠ ⌠ ⌡ ⌡ ⌡

⌠⌡

e e e

e

We used Maple to get the value of the first integral. Now, if the second integral converges it will have a finite value and so the sum of two finite values will also be finite and so the original integral will converge. Likewise, if the second integral diverges it will either be infinite or not have a value at all and adding a finite number onto this will not all of a sudden make it finite or exist and so the original integral will diverge. Therefore, this integral will converge or diverge depending only on the convergence of the second integral. As we saw in Example 5 the second integral does converge and so the whole integral must also converge. As we saw in this example, if we need to, we can split the integral up into one that doesn’t involve any problems and can be computed and one that may contain problem what we can use the Comparison Test on to determine its converge.


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Polar Coordinates

Introduction In this section we will be looking at polar coordinates. Here is a list of topics that we’ll be covering in this chapter. Polar Coordinates – We’ll introduce polar coordinates in this section. We’ll look at converting between polar coordinates and Cartesian coordinates as well as some basic graphs in polar coordinates. Tangents with Polar Coordinates – Finding tangent lines of polar curves.


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Polar Coordinates Up to this point we’ve dealt exclusively with the Cartesian (or Rectangular, or x-y) coordinate system. However, as we will see, this is not always the easiest coordinate system to work in. So, in this section we will start looking at the polar coordinate system. Coordinate systems are really nothing more than a way to define a point in space. For instance in the Cartesian coordinate system at point is given the coordinates (x,y) and we use this to define the point by starting at the origin and then moving x units horizontally followed by y units vertically. This is shown in the sketch below.

This is not, however, the only way to define a point in two dimensional space. Instead of moving vertically and horizontally from the origin to get to the point we could instead go straight out of the origin until we hit the point and then determine the angle this line makes with the positive x-axis. We could then use the distance of the point from the origin and the amount we needed to rotate from the positive x-axis as the coordinates of the point. This is shown in the sketch below.

Coordinates in this form are called polar coordinates. The above discussion may lead one to think that r must be a positive number. However, we also allow r to be negative. Below is a sketch of the two points ( )62, π and ( )62, π− .


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From this sketch we can see that if r is positive the point will be in the same quadrant as θ. On the other hand if r is negative the point will end up in the quadrant exactly opposite θ. Notice as well that the coordinates ( )62, π− describe the same point as the coordinates ( )7

62, π do. The

coordinates ( )762, π tells us to rotate an angle of 7

6π from the positive x-axis, this would put us

on the dashed line in the sketch above, and then move out a distance of 2. This leads to an important difference between Cartesian coordinates and polar coordinates. In Cartesian coordinates there is exactly one set of coordinates for any given point. With polar coordinates this isn’t true. In polar coordinates there is literally an infinite number of coordinates for a given point. For instance, the following four points are all coordinates for the same point.

5 4 25, 5, 5, 5,3 3 3 3π π π π = − = − = − −

Here is a sketch of the angles used in these four sets of coordinates.


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In the second coordinate pair we rotated in a clock-wise direction to get to the point. We shouldn’t forget about rotating in the clock-wise direction. Sometimes it’s what we have to do. The last two coordinate pairs use the fact that if we end up in the opposite quadrant from the point we can use a negative r to get back to the point and of course there is both a counter clock-wise and a clock-wise rotation to get to the angle. These four points only represent the coordinates of the point without rotating around the system more than once. If we allow the angle to make as many complete rotation about the axis system as we want then there are an infinite number of coordinates for the same point. In fact the point

( ),r θ can be represented by any of the following coordinate pairs.

( ) ( )( ), 2 , 2 1 , where is any integer.r n r n nθ π θ π+ − + + Next we should talk about the origin of the coordinate system. In polar coordinates the origin is often called the pole. Because we aren’t actually moving away from the origin/pole we know that 0r = . However, we can still rotate around the system by any angle we want and so the coordinates of the origin/pole are ( )0,θ .

Now that we’ve got a grasp on polar coordinates we need to think about converting between the two coordinate systems. Well start out with the following sketch reminding us how both coordinate systems work.

Note that we’ve got a right triangle above and with that we can get the following equations that will convert polar coordinates into Cartesian coordinates. Polar to Cartesian Conversion Formulas

cos sinx r y rθ θ= = Converting from Cartesian is almost as easy. Let’s first notice the following.

( ) ( )

( )

2 22 2

2 2 2 2

2 2 2 2

cos sin

cos sin

cos sin

x y r r

r r

r r

θ θ

θ θ

θ θ

+ = +

= +

= + =


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This is a very useful formula that we should remember, however we are after an equation for r so let’s take the square root of both sides. This gives,

2 2r x y= + Note that technically we should have a plus or minus in front of the root since we know that r can be either positive or negative. We will run with the convention of positive r here. Getting an equation for θ is almost as simple. We’ll start with,

sin tancos

y rx r

θθ

θ= =

Taking the inverse tangent of both sides gives,

1tan yx

θ − =

We will need to be careful with this because inverse tangents only return values in the range

2 2π πθ− < < . Recall that there is a second possible angle and that the second angle is given by

θ π+ . Summarizing then gives the following formulas for converting from Cartesian coordinates to polar coordinates. Cartesian to Polar Conversion Formulas

2 2 2 2 2

1tan

r x y r x yyx

θ −

= + = +

=

Let’s work a quick example. Example 1 Convert each of the following points into the given coordinate system.

(a) 24,3π −

into Cartesian coordinates. [Solution]

(b) (-1,-1) into polar coordinates. [Solution] Solution

(a) Convert 24,3π −

into Cartesian coordinates.

This conversion is easy enough. All we need to do is plug the points into the formulas.


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2 14cos 4 23 2

2 34sin 4 2 33 2

x

y

π

π

= − = − − =

= − = − = −

So, in Cartesian coordinates this point is ( )2, 2 3− .

[Return to Problems] (b) Convert (-1,-1) into polar coordinates. Let’s first get r.

( ) ( )2 21 1 2r = − + − = Now, let’s get θ.

( )1 11tan tan 11 4

πθ − −− = = = −

This is not the correct angle however. This value of θ is in the first quadrant and the point we’ve been given is in the third quadrant. As noted above we can get the correct angle by adding π onto this. Therefore, the actual angle is,

54 4π π

θ π= + =

So, in polar coordinates the point is ( )542, π . Note as well that we could have used the first θ

that we got by using a negative r. In this case the point could also be written in polar coordinates

as ( )42, π− .

[Return to Problems] We can also use the above formulas to convert equations from one coordinate system to the other. Example 2 Convert each of the following into an equation in the given coordinate system.

(a) Convert 32 5 1x x xy− = + into polar coordinates. [Solution] (b) Convert 8cosr θ= − into Cartesian coordinates. [Solution]

Solution (a) Convert 32 5 1x x xy− = + into polar coordinates. In this case there really isn’t much to do other than plugging in the formulas for x and y (i.e. the Cartesian coordinates) in terms of r and θ (i.e. the polar coordinates).

( ) ( ) ( ) ( )3

3 3 2

2 cos 5 cos 1 cos sin

2 cos 5 cos 1 cos sin

r r r r

r r r

θ θ θ θ

θ θ θ θ

− = +

− = +


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[Return to Problems] (b) Convert 8cosr θ= − into Cartesian coordinates. This one is a little trickier, but not by much. First notice that we could substitute straight for the r. However, there is no straight substitution for the cosine that will give us only Cartesian coordinates. If we had an r on the right along with the cosine then we could do a direct substitution. So, if an r on the right side would be convenient let’s put one there, just don’t forget to put on the left side as well. 2 8 cosr r θ= − We can now make some substitutions that will convert this into Cartesian coordinates. 2 2 8x y x+ = −

[Return to Problems] Before moving on to the next subject let’s do a little more work on the second part of the previous example. The equation given in the second part is actually a fairly well known graph; it just isn’t in a form that most people will quickly recognize. To identify it let’s take the Cartesian coordinate equation and do a little rearranging. 2 28 0x x y+ + = Now, complete the square on the x portion of the equation.

( )

2 2

2 2

8 16 16

4 16

x x y

x y

+ + + =

+ + =

So, this was a circle of radius 4 and center (-4,0). This leads us into the final topic of this section. Common Polar Coordinate Graphs Let’s identify a few of the more common graphs in polar coordinates. We’ll also take a look at a couple of special polar graphs. Lines Some lines have fairly simple equations in polar coordinates.

1. θ β= . We can see that this is a line by converting to Cartesian coordinates as follows


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( )

1tan

tan

tan

yxyxy x

θ β

β

β

β

−

=

=

=

=

This is a line that goes through the origin and makes an angle of β with the positive x-axis. Or, in other words it is a line through the origin with slope of tan β .

2. cosr aθ = This is easy enough to convert to Cartesian coordinates to x a= . So, this is a vertical line.

3. sinr bθ = Likewise, this converts to y b= and so is a horizontal line.

Example 3 Graph 34π

θ = , cos 4r θ = and sin 3r θ = − on the same axis system.

Solution There really isn’t too much to this one other than doing the graph so here it is.

Circles Let’s take a look at the equations of circles in polar coordinates.

1. r a= . This equation is saying that no matter what angle we’ve got the distance from the origin must be a. If you think about it that is exactly the definition of a circle of radius a centered at the origin. So, this is a circle of radius a centered at the origin. This is also one of the reasons why


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we might want to work in polar coordinates. The equation of a circle centered at the origin has a very nice equation, unlike the corresponding equation in Cartesian coordinates.

2. 2 cosr a θ= . We looked at a specific example of one of these when we were converting equations to Cartesian coordinates. This is a circle of radius a and center ( ),0a . Note that a might be negative (as it was in our example above) and so the absolute value bars are required on the radius. They should not be used however on the center.

3. 2 sinr b θ= . This is similar to the previous one. It is a circle of radius b and center ( )0,b .

4. 2 cos 2 sinr a bθ θ= + . This is a combination of the previous two and by completing the square twice it can be

shown that this is a circle of radius 2 2a b+ and center ( ),a b . In other words, this is the general equation of a circle that isn’t centered at the origin.

Example 4 Graph 7r = , 4cosr θ= , and 7sinr θ= − on the same axis system. Solution The first one is a circle of radius 7 centered at the origin. The second is a circle of radius 2

centered at (2,0). The third is a circle of radius 72

centered at 70,2

−

. Here is the graph of the

three equations.


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Note that it takes a range of 0 2θ π≤ ≤ for a complete graph of r a= and it only takes a range of 0 θ π≤ ≤ to graph the other circles given here. Cardioids and Limacons These can be broken up into the following three cases.

1. Cardioids : cosr a a θ= ± and sinr a a θ= ± . These have a graph that is vaguely heart shaped and always contain the origin.

2. Limacons with an inner loop : cosr a b θ= ± and sinr a b θ= ± with a b< . These will have an inner loop and will always contain the origin.

3. Limacons without an inner loop : cosr a b θ= ± and sinr a b θ= ± with a b> . These do not have an inner loop and do not contain the origin.

Example 5 Graph 5 5sinr θ= − , 7 6cosr θ= − , and 2 4cosr θ= + . Solution These will all graph out once in the range 0 2θ π≤ ≤ . Here is a table of values for each followed by graphs of each.

θ 5 5sinr θ= − 7 6cosr θ= − 2 4cosr θ= + 0 5 1 6

2π

0 7 2

π 5 13 -2 32π

10 7 2

2π 5 1 6


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There is one final thing that we need to do in this section. In the third graph in the previous example we had an inner loop. We will, on occasion, need to know the value of θ for which the graph will pass through the origin. To find these all we need to do is set the equation equal to zero and solve as follows,

1 2 40 2 4cos cos ,2 3 3

π πθ θ θ= + ⇒ = − ⇒ =


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Tangents with Polar Coordinates We now need to discuss some calculus topics in terms of polar coordinates. We will start with finding tangent lines to polar curves. In this case we are going to assume that the equation is in the form ( )r f θ= . With the equation in this form we can actually use the

equation for the derivative dydx

we derived when we looked at tangent lines with parametric

equations. To do this however requires us to come up with a set of parametric equations to represent the curve. This is actually pretty easy to do. From our work in the previous section we have the following set of conversion equations for going from polar coordinates to Cartesian coordinates. cos sinx r y rθ θ= = Now, we’ll use the fact that we’re assuming that the equation is in the form ( )r f θ= .

Substituting this into these equations gives the following set of parametric equations (with θ as the parameter) for the curve. ( ) ( )cos sinx f y fθ θ θ θ= = Now, we will need the following derivatives.

( ) ( ) ( ) ( )cos sin sin cos

cos sin sin cos

dx dyf f f fd d

dr drr rd d

θ θ θ θ θ θ θ θθ θ

θ θ θ θθ θ

′ ′= − = +

= − = +

The derivative dydx

is then,

Derivative with Polar Coordinates

sin cos

cos sin

dr rdy ddrdx rd

θ θθ

θ θθ

+=

−

Note that rather than trying to remember this formula it would probably be easier to remember how we derived it and just remember the formula for parametric equations. Let’s work a quick example with this.


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Example 1 Determine the equation of the tangent line to 3 8sinr θ= + at 6π

θ = .

Solution We’ll first need the following derivative.

8cosdrd

θθ

=

The formula for the derivative dydx

becomes,

( )( )2 2 2

8cos sin 3 8sin cos 16cos sin 3cos8cos 3 8sin sin 8cos 3sin 8sin

dydx

θ θ θ θ θ θ θθ θ θ θ θ θ

+ + += =

− + − −

The slope of the tangent line is,

6

3 34 3 11 323 542

dymdx πθ =

+= = =

−

Now, at 6πθ = we have 7r = . We’ll need to get the corresponding x-y coordinates so we can

get the tangent line.

7 3 77 cos 7sin6 2 6 2

x yπ π = = = =

The tangent line is then,

7 11 3 7 32 5 2

y x

= + −

For the sake of completeness here is a graph of the curve and the tangent line.


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Sequences and Series

Introduction In this chapter we’ll be taking a look at sequences and (infinite) series. Actually, this chapter will deal almost exclusively with series. However, we also need to understand some of the basics of sequences in order to properly deal with series. We will therefore, spend a little time on sequences as well. Series is one of those topics that many students don’t find all that useful. To be honest, many students will never see series outside of their calculus class. However, series do play an important role in the field of ordinary differential equations and without series large portions of the field of partial differential equations would not be possible. In other words, series is an important topic even if you won’t ever see any of the applications. Most of the applications are beyond the scope of most Calculus courses and tend to occur in classes that many students don’t take. So, as you go through this material keep in mind that these do have applications even if we won’t really be covering many of them in this class. Here is a list of topics in this chapter. Sequences – We will start the chapter off with a brief discussion of sequences. This section will focus on the basic terminology and convergence of sequences More on Sequences – Here we will take a quick look about monotonic and bounded sequences. Series – The Basics – In this section we will discuss some of the basics of infinite series. Series – Convergence/Divergence – Most of this chapter will be about the convergence/divergence of a series so we will give the basic ideas and definitions in this section. Series – Special Series – We will look at the Geometric Series, Telescoping Series, and Harmonic Series in this section.


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Sequences Let’s start off this section with a discussion of just what a sequence is. A sequence is nothing more than a list of numbers written in a specific order. The list may or may not have an infinite number of terms in them although we will be dealing exclusively with infinite sequences in this class. General sequence terms are denoted as follows,

( )

1

2

st1

first termsecond term

term

1 term

thn

n

aa

a n

a n+

−−

−

− +

M

M

Because we will be dealing with infinite sequences each term in the sequence will be followed by another term as noted above. In the notation above we need to be very careful with the subscripts. The subscript of 1n + denotes the next term in the sequence and NOT one plus the nth term! In other words, 1 1n na a+ ≠ + so be very careful when writing subscripts to make sure that the “+1” doesn’t migrate out of the subscript! This is an easy mistake to make when you first start dealing with this kind of thing. There is a variety of ways of denoting a sequence. Each of the following are equivalent ways of denoting a sequence.

{ } { } { }1 2 1 1, , , , ,n n n n na a a a a a ∞+ =

… … In the second and third notations above an is usually given by a formula. A couple of notes are now in order about these notations. First, note the difference between the second and third notations above. If the starting point is not important or is implied in some way by the problem it is often not written down as we did in the third notation. Next, we used a starting point of 1n = in the third notation only so we could write one down. There is absolutely no reason to believe that a sequence will start at 1n = . A sequence will start where ever it needs to start. Let’s take a look at a couple of sequences.


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Example 1 Write down the first few terms of each of the following sequences.

(a) 21

1

n

nn

∞

=

+

[Solution]

(b) ( ) 1

0

12

n

n

n

∞+

=

−

[Solution]

(c) { } 1n nb ∞

=, where digit of th

nb n π= [Solution] Solution

(a) 21

1

n

nn

∞

=

+

To get the first few sequence terms here all we need to do is plug in values of n into the formula given and we’ll get the sequence terms.

{{ { { {

21 1

2 53 4

1 3 4 5 62 , , , , ,4 9 16 25n n

n nn n

nn

∞

= == == =

+ =

…

Note the inclusion of the “…” at the end! This is an important piece of notation as it is the only thing that tells us that the sequence continues on and doesn’t terminate at the last term.


(b) ( ) 1

0

12

n

n

n

∞+

=

−

This one is similar to the first one. The main difference is that this sequence doesn’t start at 1n = .

( ) 1

0

1 1 1 1 11, , , , ,2 2 4 8 16

n

n

n

∞+

=

− = − − −

…

Note that the terms in this sequence alternate in signs. Sequences of this kind are sometimes called alternating sequences.


(c) { } 1n nb ∞

=, where digit of th

nb n π=

This sequence is different from the first two in the sense that it doesn’t have a specific formula for


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each term. However, it does tell us what each term should be. Each term should be the nth digit of π. So we know that 3.14159265359π = … The sequence is then,

{ }3,1, 4,1,5,9,2,6,5,3,5,… [Return to Problems]

In the first two parts of the previous example note that we were really treating the formulas as functions that can only have integers plugged into them. Or,

( ) ( ) ( ) 1

2

112

n

nnf n g nn

+−+= =

This is an important idea in the study of sequences (and series). Treating the sequence terms as function evaluations will allow us to do many things with sequences that couldn’t do otherwise. Before delving further into this idea however we need to get a couple more ideas out of the way. First we want to think about “graphing” a sequence. To graph the sequence { }na we plot the

points ( ), nn a as n ranges over all possible values on a graph. For instance, let’s graph the

sequence 21

1

n

nn

∞

=

+

. The first few points on the graph are,

( ) 3 4 5 61, 2 , 2, , 3, , 4, , 5, ,4 9 16 25

…

The graph, for the first 30 terms of the sequence, is then,

This graph leads us to an important idea about sequences. Notice that as n increases the sequence terms from our sequence terms, in this case, get closer and closer to zero. We then say that zero is the limit (or sometimes the limiting value) of the sequence and write,


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2

1lim lim 0nn n

nan→∞ →∞

+= =

This notation should look familiar to you. It is the same notation we used when we talked about the limit of a function. In fact, if you recall, we said earlier that we could think of sequences as functions in some way and so this notation shouldn’t be too surprising. Using the ideas that we developed for limits of functions we can write down the following working definition for limits of sequences. Working Definition of Limit

1. We say that lim nn

a L→∞

=

if we can make an as close to L as we want for all sufficiently large n. In other words, the value of the an’s approach L as n approaches infinity.


a→∞

= ∞

if we can make an as large as we want for all sufficiently large n. Again, in other words, the value of the an’s get larger and larger without bound as n approaches infinity.


a→∞

= −∞

if we can make an as large and negative as we want for all sufficiently large n. Again, in other words, the value of the an’s are negative and get larger and larger without bound as n approaches infinity.

The working definitions of the various sequence limits are nice in that they help us to visualize what the limit actually is. Just like with limits of functions however, there is also a precise definition for each of these limits. Let’s give those before proceeding Precise Definition of Limit

1. We say that lim nna L

→∞= if for every number 0ε > there is an integer N such that

wheneverna L n Nε− < >


a→∞

= ∞ if for every number 0M > there is an integer N such that

wheneverna M n N> > 3. We say that lim nn

a→∞

= −∞ if for every number 0M < there is an integer N such that


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wheneverna M n N< >

We won’t be using the precise definition often, but it will show up occasionally. Note that both definitions tell us that in order for a limit to exist and have a finite value all the sequence terms must be getting closer and closer to that finite value as n increases. Now that we have the definitions of the limit of sequences out of the way we have a bit of terminology that we need to look at. If lim nn

a→∞

exists and is finite we say that the sequence is

convergent. If lim nna

→∞ doesn’t exist or is infinite we say the sequence diverges. Note that

sometimes we will say the sequence diverges to ∞ if lim nna

→∞= ∞ and if lim nn

a→∞

= −∞ we will

sometimes say that the sequence diverges to −∞ . Get used to the terms “convergent” and “divergent” as we’ll be seeing them quite a bit throughout this chapter. So just how do we find the limits of sequences? Most limits of most sequences can be found using one of the following theorems. Theorem 1 Given the sequence { }na if we have a function ( )f x such that ( ) nf n a= and ( )lim

xf x L

→∞=

then lim nna L

→∞=

This theorem is basically telling us that we take the limits of sequences much like we take the limit of functions. In fact, in most cases we’ll not even really use this theorem by explicitly writing down a function. We will more often just treat the limit as if it were a limit of a function and take the limit as we always did back in Calculus I when we were taking the limits of functions. So, now that we know that taking the limit of a sequence is nearly identical to taking the limit of a function we also know that all the properties from the limits of functions will also hold. Properties

1. ( )lim lim limn n n nn n na b a b

→∞ →∞ →∞± = ±

2. lim limn nn n

ca c a→∞ →∞

=

3. ( ) ( )( )lim lim limn n n nn n na b a b

→∞ →∞ →∞=


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4. lim

lim , provided lim 0lim

nn nnn n

n nn

aa bb b

→∞

→∞ →∞→∞

= ≠

5. lim limp

pn nn n

a a→∞ →∞

= provided 0na ≥

These properties can be proved using Theorem 1 above and the function limit properties we saw in Calculus I or we can prove them directly using the precise definition of a limit using nearly identical proofs of the function limit properties. Next, just as we had a Squeeze Theorem for function limits we also have one for sequences and it is pretty much identical to the function limit version. Squeeze Theorem for Sequences If n n na c b≤ ≤ for all n N> for some N and lim limn nn n

a b L→∞ →∞

= = then lim nnc L

→∞= .

Note that in this theorem the “for all n N> for some N” is really just telling us that we need to have n n na c b≤ ≤ for all sufficiently large n, but if it isn’t true for the first few n that won’t invalidate the theorem. As we’ll see not all sequences can be written as functions that we can actually take the limit of. This will be especially true for sequences that alternate in signs. While we can always write these sequence terms as a function we simply don’t know how to take the limit of a function like that. The following theorem will help with some of these sequences. Theorem 2 If lim 0nn

a→∞

= then lim 0nna

→∞= .

Note that in order for this theorem to hold the limit MUST be zero and it won’t work for a sequence whose limit is not zero. This theorem is easy enough to prove so let’s do that. Proof of Theorem 2 The main thing to this proof is to note that, n n na a a− ≤ ≤

Then note that,

( )lim lim 0n nn na a

→∞ →∞− = − =

We then have ( )lim lim 0n nn na a

→∞ →∞− = = and so by the Squeeze Theorem we must also have,


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lim 0nna

→∞=

The next theorem is a useful theorem giving the convergence/divergence and value (for when it’s convergent) of a sequence that arises on occasion. Theorem 3

The sequence { } 0

nn

r∞

= converges if 1 1r− < ≤ and diverges for all other value of r. Also,

0 if 1 1

lim1 if 1

n

n

rr

r→∞

− < <= =

Here is a quick (well not so quick, but definitely simple) partial proof of this theorem. Partial Proof of Theorem 3 We’ll do this by a series of cases although the last case will not be completely proven. Case 1 : 1r > We know from Calculus I that lim x

xr

→∞= ∞ if 1r > and so by Theorem 1 above we also know

that lim n

nr

→∞= ∞ and so the sequence diverges if 1r > .

Case 2 : 1r = In this case we have, lim lim1 lim1 1n n

n n nr

→∞ →∞ →∞= = =

So, the sequence converges for 1r = and in this case its limit is 1. Case 3 : 0 1r< < We know from Calculus I that lim 0x

xr

→∞= if 0 1r< < and so by Theorem 1 above we also know

that lim 0n

nr

→∞= and so the sequence converges if 0 1r< < and in this case its limit is zero.

Case 4 : 0r = In this case we have, lim lim 0 lim 0 0n n

n n nr

→∞ →∞ →∞= = =

So, the sequence converges for 0r = and in this case its limit is zero.


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Case 5 : 1 0r− < < First let’s note that if 1 0r− < < then 0 1r< < then by Case 3 above we have,

lim lim 0nn

n nr r

→∞ →∞= =

Theorem 2 above now tells us that we must also have, lim 0n

nr

→∞= and so if 1 0r− < < the

sequence converges and has a limit of 0. Case 6 : 1r = − In this case the sequence is,

{ } ( ){ } { } 00 01 1, 1,1, 1,1, 1,1, 1,nn

nn nr

∞∞ ∞

== == − = − − − − …

and hopefully it is clear that ( )lim 1 n

n→∞− doesn’t exist. Recall that in order of this limit to exist the

terms must be approaching a single value as n increases. In this case however the terms just alternate between 1 and -1 and so the limit does not exist. So, the sequence diverges for 1r = − . Case 7 : 1r < − In this case we’re not going to go through a complete proof. Let’s just see what happens if we let

2r = − for instance. If we do that the sequence becomes,

{ } ( ){ } { } 00 02 1, 2,4, 8,16, 32,nn

nn nr

∞∞ ∞

== == − = − − − …

So, if 2r = − we get a sequence of terms whose values alternate in sign and get larger and larger

and so ( )lim 2 n

n→∞− doesn’t exist. It does not settle down to a single value as n increases nor do the

terms ALL approach infinity. So, the sequence diverges for 2r = − . We could do something similar for any value of r such that 1r < − and so the sequence diverges for 1r < − .

Let’s take a look at a couple of examples of limits of sequences. Example 2 Determine if the following sequences converge or diverge. If the sequence converges determine its limit.

(a) 2

22

3 110 5 n

nn n

∞

=

− +

[Solution]


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(b) 2

1

n

nn

∞

=

e [Solution]

(c) ( )

1

1 n

nn

∞

=

−

[Solution]

(d) ( ){ }0

1 n

n

∞

=− [Solution]

Solution

(a) 2

22

3 110 5 n

nn n

∞

=

− +

In this case all we need to do is recall the method that was developed in Calculus I to deal with the limits of rational functions. See the Limits At Infinity, Part I section of my Calculus I notes for a review of this if you need to. To do a limit in this form all we need to do is factor from the numerator and denominator the largest power of n, cancel and then take the limit.

22 2 2

22

1 13 33 1 3lim lim lim 0010 5 555n n n

nn n nn n n

nn→∞ →∞ →∞

− − − = = =11+ ++

So the sequence converges and its limit is 3

5 . [Return to Problems]

(b) 2

1

n

nn

∞

=

e

We will need to be careful with this one. We will need to use L’Hospital’s Rule on this sequence. The problem is that L’Hospital’s Rule only works on functions and not on sequences. Normally this would be a problem, but we’ve got Theorem 1 from above to help us out. Let’s define

( )2x

f xx

=e

and note that,

( )2n

f nn

=e


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Theorem 1 says that all we need to do is take the limit of the function.

2 2 22lim lim lim

1

n x x

n x xn x→∞ →∞ →∞= = = ∞

e e e

So, the sequence in this part diverges (to ∞ ). More often than not we just do L’Hospital’s Rule on the sequence terms without first converting to x’s since the work will be identical regardless of whether we use x or n. However, we really should remember that technically we can’t do the derivatives while dealing with sequence terms.


(c) ( )

1

1 n

nn

∞

=

−

We will also need to be careful with this sequence. We might be tempted to just say that the limit of the sequence terms is zero (and we’d be correct). However, technically we can’t take the limit of sequences whose terms alternate in sign, because we don’t know how to do limits of functions that exhibit that same behavior. Also, we want to be very careful to not rely too much on intuition with these problems. As we will see in the next section, and in later sections, our intuition can lead us astray in these problem if we aren’t careful. So, let’s work this one by the book. We will need to use Theorem 2 on this problem. To this we’ll first need to compute,

( )1 1lim lim 0n

n nn n→∞ →∞

−= =

Therefore, since the limit of the sequence terms with absolute value bars on them goes to zero we know by Theorem 2 that,

( )1lim 0

n

n n→∞

−=

which also means that the sequence converges to a value of zero. [Return to Problems]

(d) ( ){ }0

1 n

n

∞

=−

For this theorem note that all we need to do is realize that this is the sequence in Theorem 3 above using 1r = − . So, by Theorem 3 this sequence diverges.



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We now need to give a warning about misusing Theorem 2. Theorem 2 only works if the limit is zero. If the limit of the absolute value of the sequence terms is not zero then the theorem will not hold. The last part of the previous example is a good example of this (and in fact this warning the whole reason that part is there). Notice that

( )lim 1 lim1 1n

n n→∞ →∞− = =

and yet, ( )lim 1 n

n→∞− doesn’t even exist let alone equal 1. So, be careful using this Theorem 2.

You must always remember that it only works if the limit is zero. Before moving onto the next section we need to give one more theorem that we’ll need for a proof down the road. Theorem 4 For the sequence { }na if both 2lim nn

a L→∞

= and 2 1lim nna L+→∞

= then { }na is convergent and

lim nna L

→∞= .

Proof of Theorem 4 Let 0ε > . Then since 2lim nn

a L→∞

= there is an 1 0N > such that if 1n N> we know that,

2na L ε− <

Likewise, because 2 1lim nna L+→∞

= there is an 2 0N > such that if 2n N> we know that,

2 1na L ε+ − <

Now, let { }1 2max 2 , 2 1N N N= + and let n N> . Then either 2n ka a= for some 1k N> or

2 1n ka a += for some 2k N> and so in either case we have that,

na L ε− <

Therefore, lim nn

a L→∞

= and so { }na is convergent.


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More on Sequences In the previous section we introduced the concept of a sequence and talked about limits of sequences and the idea of convergence and divergence for a sequence. In this section we want to take a quick look at some ideas involving sequences. Let’s start off with some terminology and definitions. Given any sequence { }na we have the following.

1. We call the sequence increasing if 1n na a +< for every n.

2. We call the sequence decreasing if 1n na a +> for every n.

3. If { }na is an increasing sequence or { }na is a decreasing sequence we call it monotonic.

4. If there exists a number m such that nm a≤ for every n we say the sequence is bounded below. The number m is sometimes called a lower bound for the sequence.

5. If there exists a number M such that na M≤ for every n we say the sequence is bounded above. The number M is sometimes called an upper bound for the sequence.

6. If the sequence is both bounded below and bounded above we call the sequence bounded.

Note that in order for a sequence to be increasing or decreasing it must be increasing/decreasing for every n. In other words, a sequence that increases for three terms and then decreases for the rest of the terms is NOT a decreasing sequence! Also note that a monotonic sequence must always increase or it must always decrease. Before moving on we should make a quick point about the bounds for a sequence that is bounded above and/or below. We’ll make the point about lower bounds, but we could just as easily make it about upper bounds. A sequence is bounded below if we can find any number m such that nm a≤ for every n. Note however that if we find one number m to use for a lower bound then any number smaller than m will also be a lower bound. Also, just because we find one lower bound that doesn’t mean there won’t be a “better” lower bound for the sequence than the one we found. In other words, there are an infinite number of lower bounds for a sequence that is bounded below, some will be better than others. In my class all that I’m after will be a lower bound. I don’t necessarily need the best lower bound, just a number that will be a lower bound for the sequence. Let’s take a look at a couple of examples.


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Example 1 Determine if the following sequences are monotonic and/or bounded.

(a) { }2

0nn

∞

=− [Solution]

(b) ( ){ }1

11 n

n

∞+

=− [Solution]

(c) 25

2

nn

∞

=

[Solution]

Solution

(a) { }2

0nn

∞

=−

This sequence is a decreasing sequence (and hence monotonic) because,

( )22 1n n− > − + for every n. Also, since the sequence terms will be either zero or negative this sequence is bounded above. We can use any positive number or zero as the bound, M, however, it’s standard to choose the smallest possible bound if we can and it’s a nice number. So, we’ll choose 0M = since,

2 0 for every n n− ≤ This sequence is not bounded below however since we can always get below any potential bound by taking n large enough. Therefore, while the sequence is bounded above it is not bounded. As a side note we can also note that this sequence diverges (to −∞ if we want to be specific).


(b) ( ){ }1

11 n

n

∞+

=−

The sequence terms in this sequence alternate between 1 and -1 and so the sequence is neither an increasing sequence or a decreasing sequence. Since the sequence is neither an increasing nor decreasing sequence it is not a monotonic sequence. The sequence is bounded however since it is bounded above by 1 and bounded below by -1. Again, we can note that this sequence is also divergent.



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(c) 25

2

nn

∞

=

This sequence is a decreasing sequence (and hence monotonic) since,

( )22

2 21n n

>+

The terms in this sequence are all positive and so it is bounded below by zero. Also, since the sequence is a decreasing sequence the first sequence term will be the largest and so we can see that the sequence will also be bounded above by 2

25 . Therefore, this sequence is bounded. We can also take a quick limit and note that this sequence converges and its limit is zero.

[Return to Problems] Now, let’s work a couple more examples that are designed to make sure that we don’t get too used to relying on our intuition with these problems. As we noted in the previous section our intuition can often lead us astray with some of the concepts we’ll be looking at in this chapter. Example 2 Determine if the following sequences are monotonic and/or bounded.

(a) 11 n

nn

∞

=

+

[Solution]

(b) 3

4010000 n

nn

∞

=

+

[Solution]

Solution

(a) 11 n

nn

∞

=

+

We’ll start with the bounded part of this example first and then come back and deal with the increasing/decreasing question since that is where students often make mistakes with this type of sequence. First, n is positive and so the sequence terms are all positive. The sequence is therefore bounded below by zero. Likewise each sequence term is the quotient of a number divided by a larger number and so is guaranteed to be less that one. The sequence is then bounded above by one. So, this sequence is bounded. Now let’s think about the monotonic question. First, students will often make the mistake of assuming that because the denominator is larger the quotient must be decreasing. This will not always be the case and in this case we would be wrong. This sequence is increasing as we’ll see.


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To determine the increasing/decreasing nature of this sequence we will need to resort to Calculus I techniques. First consider the following function and its derivative.

( ) ( )( )2

11 1

xf x f xx x

′= =+ +

We can see that the first derivative is always positive and so from Calculus I we know that the function must then be an increasing function. So, how does this help us? Notice that,

( )1 n

nf n an

= =+

Therefore because 1n n< + and ( )f x is increasing we can also say that,

( ) ( ) 1 111

1 1 1n n n nn na f n f n a a a

n n + +

+= = < + = = ⇒ <

+ + +

In other words, the sequence must be increasing. Note that now that we know the sequence is an increasing sequence we can get a better lower bound for the sequence. Since the sequence is increasing the first term in the sequence must be the smallest term and so since we are starting at 1n = we could also use a lower bound of 1

2 for this sequence. It is important to remember that any number that is always less than or equal to all the sequence terms can be a lower bound. Some are better than others however. A quick limit will also tell us that this sequence converges with a limit of 1. Before moving on to the next part there is a natural question that many students will have at this point. Why did we use Calculus to determine the increasing/decreasing nature of the sequence when we could have just plugged in a couple of n’s and quickly determined the same thing? The answer to this question is the next part of this example!


(b) 3

4010000 n

nn

∞

=

+

This is a messy looking sequence, but it needs to be in order to make the point of this part. First, notice that, as with the previous part, the sequence terms are all positive and will all be less than one (since the numerator is guaranteed to be less than the denominator) and so the sequence is bounded.


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Now, let’s move on to the increasing/decreasing question. As with the last problem, many students will look at the exponents in the numerator and denominator and determine based on that that sequence terms must decrease. This however, isn’t a decreasing sequence. Let’s take a look at the first few terms to see this.

1 2

3 4

5 6

7 8

9 10

1 10.00009999 0.000798710001 1252

27 40.005678 0.00624010081 6411 270.011756 0.01912285 1412343 320.02766 0.03632

12401 881729 10.04402 0.05

16561 20

a a

a a

a a

a a

a a

= ≈ = ≈

= ≈ = ≈

= ≈ = ≈

= ≈ = ≈

= ≈ = =

The first 10 terms of this sequence are all increasing and so clearly the sequence can’t be a decreasing sequence. Recall that a sequence can only be decreasing if ALL the terms are decreasing. Now, we can’t make another common mistake and assume that because the first few terms increase then whole sequence must also increase. If we did that we would also be mistaken as this is also not an increasing sequence. This sequence is neither decreasing or increasing. The only sure way to see this is to do the Calculus I approach to increasing/decreasing functions. In this case we’ll need the following function and its derivative.

( ) ( ) ( )( )

2 43

24 4

3000010000 10000

x xxf x f xx x

− −′= =

+ +

This function will have the following three critical points,

4 40, 30000 13.1607, 30000 13.1607x x x= = ≈ = − ≈ − Why critical points? Remember these are the only places where the function may change sign! Our sequence starts at 0n = and so we can ignore the third one since it lies outside the values of n that we’re considering. By plugging in some test values of x we can quickly determine that the

derivative is positive for 40 30000 13.16x< < ≈ and so the function is increasing in this range.

Likewise, we can see that the derivative is negative for 4 30000 13.16x > ≈ and so the function will be decreasing in this range.


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So, our sequence will be increasing for 0 13n≤ ≤ and decreasing for 13n ≥ . Therefore the function is not monotonic. Finally, note that this sequence will also converge and has a limit of zero.

[Return to Problems] So, as the last example has shown we need to be careful in making assumptions about sequences. Our intuition will often not be sufficient to get the correct answer and we can NEVER make assumptions about a sequence based on the value of the first few terms. As the last part has shown there are sequences which will increase or decrease for a few terms and then change direction after that. Note as well that we said “first few terms” here, but it is completely possible for a sequence to decrease for the first 10,000 terms and then start increasing for the remaining terms. In other words, there is no “magical” value of n for which all we have to do is check up to that point and then we’ll know what the whole sequence will do. The only time that we’ll be able to avoid using Calculus I techniques to determine the increasing/decreasing nature of a sequence is in sequences like part (c) of Example 1. In this case increasing n only changed (in fact increased) the denominator and so we were able to determine the behavior of the sequence based on that. In Example 2 however, increasing n increased both the denominator and the numerator. In cases like this there is no way to determine which increase will “win out” and cause the sequence terms to increase or decrease and so we need to resort to Calculus I techniques to answer the question. We’ll close out this section with a nice theorem that we’ll use in some of the proofs later in this chapter. Theorem If { }na is bounded and monotonic then { }na is convergent.

Be careful to not misuse this theorem. It does not say that if a sequence is not bounded and/or not monotonic that it is divergent. Example 2b is a good case in point. The sequence in that example was not monotonic but it does converge. Note as well that we can make several variants of this theorem. If { }na is bounded above and

increasing then it converges and likewise if { }na is bounded below and decreasing then it

converges.


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Series – The Basics In this section we will introduce the topic that we will be discussing for the rest of this chapter. That topic is infinite series. So just what is a infinite series? Well, let’s start with a sequence

{ } 1n na ∞

= (note the 1n = is for convenience, it can be anything) and define the following,

1 1

2 1 2

3 1 2 3

4 1 2 3 4

1 2 3 41

n

n n ii

s as a as a a as a a a a

s a a a a a a=

== += + += + + +

= + + + + + = ∑

M

L

The ns are called partial sums and notice that they will form a sequence, { } 1n ns ∞

=. Also recall

that the Σ is used to represent this summation and called a variety of names. The most common names are : series notation, summation notation, and sigma notation. You should have seen this notation, at least briefly, back when you saw the definition of a definite integral in Calculus I. If you need a quick refresher on summation notation see the review of summation notation in my Calculus I notes.

Now back to series. We want to take a look at the limit of the sequence of partial sums, { } 1n ns ∞

=.

Notationally we’ll define,

1 1

lim limn

n i in n i is a a

∞

→∞ →∞= =

= =∑ ∑

We will call 1

ii

a∞

=∑ an infinite series and note that the series “starts” at 1i = because that is

where our original sequence, { } 1n na ∞

=, started. Had our original sequence started at 2 then our

infinite series would also have started at 2. The infinite series will start at the same value that the sequence of terms (as opposed to the sequence of partial sums) starts.

If the sequence of partial sums, { } 1n ns ∞

=, is convergent and its limit is finite then we also call the

infinite series, 1

ii

a∞

=∑ convergent and if the sequence of partial sums is diverent then the infinite

series is also called divergent. Note that sometimes it is convenient to write the infinite series as,


Math 140


1 2 31

i ni

a a a a a∞

=

= + + + + +∑ L L

We do have to be careful with this however. This implies that an infinite series is just an infinite sum of terms and as well see in the next section this is not really true. In the next section we’re going to be discussing in greater detail the value of an infinite series, provided it has one of course as well as the ideas of convergence and divergence. This section is going to be devoted mostly to notational issues as well as making sure we can do some basic manipulations with infinite series so we are ready for them when we need to be able to deal with them in later sections. First, we should note that in most of this chapter we will refer to infinite series as simply series. If we ever need to work with both infinite and finite series we’ll be more careful with terminology, but in most sections we’ll be dealing exclusively with infinite series and so we’ll just call them series.

Now, in 1

ii

a∞

=∑ the i is called the index of summation or just index for short and note that the

letter we use to represent the index does not matter. So for example the following series are all the same. The only difference is the letter we’ve used for the index.

2 2 20 0 0

3 3 3 .1 1 1i k n

etci k n

∞ ∞ ∞

= = =

= =+ + +∑ ∑ ∑

It is important to again note that the index will start at whatever value the sequence of series terms starts at and this can literally be anything. So far we’ve used 0n = and 1n = but the index could have started anywhere. In fact, we will usually use na∑ to represent an infinite

series in which the starting point for the index is not important. When we drop the initial value of the index we’ll also drop the infinity from the top so don’t forget that it is still technically there. We will be dropping the initial value of the index in quite a few facts and theorems that we’ll be seeing throughout this chapter. In these facts/theorems the starting point of the series will not affect the result and so to simplify the notation and to avoid giving the impression that the starting point is important we will drop the index from the notation. Do not forget however, that there is a starting point and that this will be an infinite series. Note however, that if we do put an initial value of the index on a series in a fact/theorem it is there because it really does need to be there.


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Now that some of the notational issues are out of the way we need to start thinking about various ways that we can manipulate series. We’ll start this off with basic arithmetic with infinite series as we’ll need to be able to do that on occasion. We have the following properties. Properties If na∑ and nb∑ are both convergent series then,

6. nca∑ , where c is any number, is also convergent and

n nca c a=∑ ∑

7. n nn k n k

a b∞ ∞

= =

±∑ ∑ is also convergent and,

( )n n n nn k n k n k

a b a b∞ ∞ ∞

= = =

± = ±∑ ∑ ∑ .

The first property is simply telling us that we can always factor a multiplicative constant out of an infinite series and again recall that if we don’t put in an initial value of the index that the series can start at any value. Also recall that in these cases we won’t put an infinity at the top either. The second property says that if we add/subtract series all we really need to do is add/subtract the series terms. Note as well that in order to add/subtract series we need to make sure that both have the same initial value of the index and the new series will also start at this value. Before we move on to a different topic let’s discuss multiplication of series briefly. We’ll start both series at 0n = for a later formula and then note that,

( )0 0 0

n n n nn n n

a b a b∞ ∞ ∞

= = =

≠ ∑ ∑ ∑

To convince yourself that this isn’t true consider the following product of two finite sums.

( ) ( )2 2 32 3 5 6 7 3x x x x x x+ − + = − − + Yeah, it was just the multiplication of two polynomials. Each is a finite sum and so it makes the point. In doing the multiplication we didn’t just multiply the constant terms, then the x terms, etc. Instead we had to distribute the 2 through the second polynomial, then distribute the x through the second polynomial and finally combine like terms.


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Multiplying infinite series (even though we said we can’t think of an infinite series as an infinite sum) needs to be done in the same manner. With multiplication we’re really asking us to do the following,

( )( )0 1 2 3 0 1 2 30 0

n nn n

a b a a a a b b b b∞ ∞

= =

= + + + + + + + + ∑ ∑ L L

To do this multiplication we would have to distribute the a0 through the second term, distribute the a1 through, etc then combine like terms. This is pretty much impossible since both series have an infinite set of terms in them, however the following formula can be used to determine the product of two series.

0 0 0

n n nn n n

a b c∞ ∞ ∞

= = =

= ∑ ∑ ∑ where

0

n

n i n ii

c a b −=

= ∑

We also can’t say a lot about the convergence of the product. Even if both of the original series are convergent it is possible for the product to be divergent. The reality is that multiplication of series is a somewhat difficult process and in general is avoided if possible. We will take a brief look at it towards the end of the chapter when we’ve got more work under our belt and we run across a situation where it might actually be what we want to do. Until then, don’t worry about multiplying series. The next topic that we need to discuss in this section is that of index shift. To be honest this is not a topic that we’ll see all that often in this course. In fact, we’ll use it once in the next section and then not use it again in all likelihood. Despite the fact that we won’t use it much in this course doesn’t mean however that it isn’t used often in other classes where you might run across series. So, we will cover it briefly here so that you can say you’ve seen it. The basic idea behind index shifts is to start a series at a different value for whatever the reason (and yes, there are legitimate reasons for doing that). Consider the following series,

2

52n

n

n∞

=

+∑

Suppose that for some reason we wanted to start this series at 0n = , but we didn’t want to change the value of the series. This means that we can’t just change the 2n = to 0n = as this would add in two new terms to the series and thus changing its value. Performing an index shift is a fairly simple process to do. We’ll start by defining a new index, say i, as follows, 2i n= −


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Now, when 2n = , we will get 0i = . Notice as well that if n = ∞ then 2i = ∞ − = ∞ , so only the lower limit will change here. Next, we can solve this for n to get, 2n i= + We can now completely rewrite the series in terms of the index i instead of the index n simply by plugging in our equation for n in terms of i.

( )2 2

2 0 0

2 55 72 2 2n i i

n i i

in i∞ ∞ ∞

+ += = =

+ ++ += =∑ ∑ ∑

To finish the problem out we’ll recall that the letter we used for the index doesn’t matter and so we’ll change the final i back into an n to get,

22 0

5 72 2n n

n n

n n∞ ∞

+= =

+ +=∑ ∑

To convince yourselves that these really are the same summation let’s write out the first couple of terms for each of them,

2 3 4 5

2

2 2 3 4 50

5 7 8 9 102 2 2 2 2

7 7 8 9 102 2 2 2 2

nn

nn

n

n

∞

=

∞

+=

+= + + + +

+= + + + +

∑

∑

L

L

So, sure enough the two series do have exactly the same terms. There is actually an easier way to do an index shift. The method given above is the technically correct way of doing an index shift. However, notice in the above example we decreased the initial value of the index by 2 and all the n’s in the series terms increased by 2 as well. This will always work in this manner. If we decrease the initial value of the index by a set amount then all the other n’s in the series term will increase by the same amount. Likewise, if we increase the initial value of the index by a set amount, then all the n’s in the series term will decrease by the same amount. Let’s do a couple of examples using this shorthand method for doing index shifts. Example 1 Perform the following index shifts.

(a) Write 1

1

n

nar

∞−

=∑ as a series that starts at 0n = .

(b) Write 2

11 1 3n

n

n∞

+= −∑ as a series that starts at 3n = .

Solution (a) In this case we need to decrease the initial value by 1 and so the n’s (okay the single n) in the


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term must increase by 1 as well.

( )1 11

1 0 0

nn n

n n nar ar ar

∞ ∞ ∞+ −−

= = =

= =∑ ∑ ∑

(b) For this problem we want to increase the initial value by 2 and so all the n’s in the series term must decrease by 2.

( )( )

( )2 22

1 12 11 3 3

2 21 3 1 31 3n nn

n n n

n nn∞ ∞ ∞

+ −− += = =

− −= =

− −−∑ ∑ ∑

The final topic in this section is again a topic that we’ll not be seeing all that often in this class, although we will be seeing it more often than the index shifts. This final topic is really more about alternate ways to write series when the situation requires it. Let’s start with the following series and note that the 1n = starting point is only for convenience since we need to start the series somewhere.

1 2 3 4 51

nn

a a a a a a∞

=

= + + + + +∑ L

Notice that if we ignore the first term the remaining terms will also be a series that will start at

2n = instead of 1n = So, we can rewrite the original series as follows,

11 2

n nn n

a a a∞ ∞

= =

= +∑ ∑

In this example we say that we’ve stripped out the first term. We could have stripped out more terms if we wanted to. In the following series we’ve stripped out the first two terms and the first four terms respectively.

1 2

1 3

1 2 3 41 5

n nn n

n nn n

a a a a

a a a a a a

∞ ∞

= =

∞ ∞

= =

= + +

= + + + +

∑ ∑

∑ ∑

Being able to strip out terms will, on occasion, simplify our work or allow us to reuse a prior result so it’s an important idea to remember. Notice that in the second example above we could have also denoted the four terms that we stripped out as a finite series as follows,

4

1 2 3 41 5 1 5

n n n nn n n n

a a a a a a a a∞ ∞ ∞

= = = =

= + + + + = +∑ ∑ ∑ ∑


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This is a convenient notation when we are stripping out a large number of terms or if we need to strip out an undetermined number of terms. In general, we can write a series as follows,

1 1 1

N

n n nn n n N

a a a∞ ∞

= = = +

= +∑ ∑ ∑

We’ll leave this section with an important warning about terminology. Don’t get sequences and series confused! A sequence is a list of numbers written in a specific order while an infinite series is a limit of a sequence of finite series and hence, if it exists will be a single value. So, once again, a sequence is a list of numbers while a series is a single number, provided it makes sense to even compute the series. Students will often confuse the two and try to use facts pertaining to one on the other. However, since they are different beasts this just won’t work. There will be problems where we are using both sequences and series so we’ll always have to remember that they are different.


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Series – Convergence/Divergence In the previous section we spent some time getting familiar with series and we briefly defined convergence and divergence. Before worrying about convergence and divergence of a series we wanted to make sure that we’ve started to get comfortable with the notation involved in series and some of the various manipulations of series that we will, on occasion, need to be able to do. As noted in the previous section most of what we were doing there won’t be done much in this chapter. So, it is now time to start talking about the convergence and divergence of a series as this will be a topic that we’ll be dealing with to one extent of another in almost all of the remaining sections of this chapter. So, let’s recap just what an infinite series is and what it means for a series to be convergent or

divergent. We’ll start with a sequence { } 1n na ∞

= and again note that we’re starting the sequence at

1n = only for the sake of convenience and it can, in fact, be anything. Next we define the partial sums of the series as,

1 1

2 1 2

3 1 2 3

4 1 2 3 4

1 2 3 41

n

n n ii

s as a as a a as a a a a

s a a a a a a=

== += + += + + +

= + + + + + = ∑

M

L

and these form a new sequence, { } 1n ns ∞

=.

An infinite series, or just series here since almost every series that we’ll be looking at will be an infinite series, is then the limit of the partial sums. Or,

1

limi nnia s

∞

→∞=

=∑

If the sequence of partial sums is a convergent sequence (i.e. its limit exists and is finite) then the

series is also called convergent and in this case if lim nns s

→∞= then,

1i

ia s

∞

=

=∑ . Likewise, if the

sequence of partial sums is a divergent sequence (i.e. its limit doesn’t exist or is plus or minus infinity) then the series is also called divergent. Let’s take a look at some series and see if we can determine if they are convergent or divergent and see if we can determine the value of any convergent series we find.


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Example 1 Determine if the following series is convergent or divergent. If it converges determine its value.

1nn

∞

=∑

Solution To determine if the series is convergent we first need to get our hands on a formula for the general term in the sequence of partial sums.

1

n

ni

s i=

= ∑

This is a known series and its value can be shown to be,

( )1

12

n

ni

n ns i

=

+= =∑

Don’t worry if you didn’t know this formula (I’d be surprised if anyone knew it…) as you won’t be required to know it in my course. So, to determine if the series is convergent we will first need to see if the sequence of partial sums,

( )1

12

n

n n∞

=

+

is convergent or divergent. That’s not terribly difficult in this case. The limit of the sequence terms is,

( )1lim

2n

n n→∞

+= ∞

Therefore, the sequence of partial sums diverges to ∞ and so the series also diverges. So, as we saw in this example we had to know a fairly obscure formula in order to determine the convergence of this series. In general finding a formula for the general term in the sequence of partial sums is a very difficult process. In fact after the next section we’ll not be doing much with the partial sums of series due to the extreme difficulty faced in finding the general formula. This also means that we’ll not be doing much work with the value of series since in order to get the value we’ll also need to know the general formula for the partial sums. We will continue with a few more examples however, since this is technically how we determine convergence and the value of a series. Also, the remaining examples we’ll be looking at in this section will lead us to a very important fact about the convergence of series.


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So, let’s take a look at a couple more examples. Example 2 Determine if the following series converges or diverges. If it converges determine its sum.

22

11n n

∞

= −∑

Solution This is actually one of the few series in which we are able to determine a formula for the general term in the sequence of partial fractions. However, in this section we are more interested in the general idea of convergence and divergence and so we’ll put off discussing the process for finding the formula until the next section. The general formula for the partial sums is,

( )2

2

1 3 1 11 4 2 2 1

n

ni

si n n=

= = − −− +∑

and in this case we have,

( )

3 1 1 3lim lim4 2 2 1 4nn n

sn n→∞ →∞

= − − = +

The sequence of partial sums converges and so the series converges also and its value is,

22

1 31 4n n

∞

=

=−∑

Example 3 Determine if the following series converges or diverges. If it converges determine its sum.

( )0

1 n

n

∞

=

−∑

Solution In this case we really don’t need a general formula for the partial sums to determine the convergence of this series. Let’s just write down the first few partial sums.

0

1

2

3

11 1 01 1 1 11 1 1 1 0

.

ssssetc

== − == − + == − + − =


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So, it looks like the sequence of partial sums is,

{ } { }0 1,0,1,0,1,0,1,0,1,n ns ∞

== …

and this sequence diverges since lim nns

→∞ doesn’t exist. Therefore, the series also diverges.

Example 4 Determine if the following series converges or diverges. If it converges determine its sum.

11

13n

n

∞

−=

∑

Solution Here is the general formula for the partial sums for this series.

11

1 3 113 2 3

n

n i ni

s −=

= = −

∑

Again, do not worry about knowing this formula. This is not something that you’ll ever be asked to know in my class. In this case the limit of the sequence of partial sums is,

3 1 3lim lim 12 3 2n nn n

s→∞ →∞

= − =

The sequence of partial sums is convergent and so the series will also be convergent. The value of the series is,

11

1 33 2n

n

∞

−=

=∑

As we already noted, do not get excited about determining the general formula for the sequence of partial sums. There is only going to be one type of series where you will need to determine this formula and the process in that case isn’t too bad. In fact, you already know how to do most of the work in the process as you’ll see in the next section. So, we’ve determined the convergence of four series now. Two of the series converged and two diverged. Let’s go back and examine the series terms for each of these. For each of the series let’s take the limit as n goes to infinity of the series terms (not the partial sums!!).


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( )

2

1

lim this series diverged

1lim 0 this series converged1

lim 1 doesn't exist this series diverged

1lim 0 this series converged3

n

n

n

n

nn

n

n

→∞

→∞

→∞

−→∞

= ∞

=−

−

=

Notice that for the two series that converged the series term itself was zero in the limit. This will always be true for convergent series and leads to the following theorem. Theorem If na∑ converges then lim 0nn

a→∞

= .

Proof First let’s suppose that the series starts at 1n = . If it doesn’t then we can modify things as appropriate below. Then the partial sums are,

1

1 1 2 3 4 1 1 2 3 4 11 1

n n

n i n n i n ni i

s a a a a a a s a a a a a a a−

− − −= =

= = + + + + + = = + + + + + +∑ ∑L L

Next, we can use these two partial sums to write, 1n n na s s −= −

Now because we know that na∑ is convergent we also know that the sequence { } 1n ns ∞

= is also

convergent and that lim nns s

→∞= for some finite value s. However, since 1n − → ∞ as n → ∞ we

also have 1lim nns s−→∞

= .

We now have, ( )1 1lim lim lim lim 0n n n n nn n n n

a s s s s s s− −→∞ →∞ →∞ →∞= − = − = − =

Be careful to not misuse this theorem! This theorem gives us a requirement for convergence but not a guarantee of convergence. In other words, the converse is NOT true. If lim 0nn

a→∞

= the

series may actually diverge! Consider the following two series.

21 1

1 1n nn n

∞ ∞

= =∑ ∑


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In both cases the series terms are zero in the limit as n goes to infinity, yet only the second series converges. The first series diverges. It will be a couple of sections before we can prove this, so at this point please believe this and know that you’ll be able to prove the convergence of these two series in a couple of sections. Again, as noted above, all this theorem does is give us a requirement for a series to converge. In order for a series to converge the series terms must go to zero in the limit. If the series terms do not go to zero in the limit then there is no way the series can converge since this would violate the theorem. This leads us to the first of many tests for the convergence/divergence of a series that we’ll be seeing in this chapter. Divergence Test If lim 0nn

a→∞

≠ then na∑ will diverge. Again, do NOT misuse this test. This test only says that a series is guaranteed to diverge if the series terms don’t go to zero in the limit. If the series terms do happen to go to zero the series may or may not converge! Again, recall the following two series,

1

21

1 diverges

1 converges

n

n

n

n

∞

=

∞

=

∑

∑

One of the more common mistakes that students make when they first get into series is to assume that if lim 0nn

a→∞

= then na∑ will converge. There is just no way to guarantee this so be careful!

Let’s take a quick look at an example of how this test can be used. Example 5 Determine if the following series is convergent or divergent.

2 3

30

410 2n

n nn

∞

=

−+∑

Solution With almost every series we’ll be looking at in this chapter the first thing that we should do is take a look at the series terms and see if they go to zero of not. If it’s clear that the terms don’t go to zero use the Divergence Test and be done with the problem. That’s what we’ll do here.

2 3

3

4 1lim 010 2 2n

n nn→∞

−= − ≠

+


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The limit of the series terms isn’t zero and so by the Divergence Test the series diverges. The divergence test is the first test of many tests that we will be looking at over the course of the next several sections. You will need to keep track of all these tests, the conditions under which they can be used and their conclusions all in one place so you can quickly refer back to them as you need to. Next we should talk briefly revisit arithmetic of series and convergence/divergence. As we saw in the previous section if na∑ and nb∑ are both convergent series then so are nca∑ and

( )n nn k

a b∞

=

±∑ . Furthermore, these series will have the following sums or values.

( )n n n n n nn k n k n k

ca c a a b a b∞ ∞ ∞

= = =

= ± = ±∑ ∑ ∑ ∑ ∑

We’ll see an example of this in the next section after we get a few more examples under our belt. At this point just remember that a sum of convergent sequences is convergent and multiplying a convergent sequence by a number will not change its convergence. We need to be a little careful with these facts when it comes to divergent series. In the first case if na∑ is divergent then nca∑ will also be divergent (provided c isn’t zero of course) since

multiplying a series that is infinite in value or doesn’t have a value by a finite value (i.e. c) won’t change the fact that the series has an infinite or no value. However, it is possible to have both

na∑ and nb∑ be divergent series and yet have ( )n nn k

a b∞

=

±∑ be a convergent series.

Now, since the main topic of this section is the convergence of a series we should mention a stronger type of convergence. A series na∑ is said to converge absolutely if na∑ also

converges. Absolute convergence is stronger than convergence in the sense that a series that is absolutely convergent will also be convergent, but a series that is convergent may or may not be absolutely convergent. In fact if na∑ converges and na∑ diverges the series na∑ is called conditionally

convergent. At this point we don’t really have the tools at hand to properly investigate this topic in detail nor do we have the tools in hand to determine if a series is absolutely convergent or not. So we’ll not say anything more about this subject for a while. When we finally have the tools in hand to discuss this topic in more detail we will revisit it. Until then don’t worry about it. The idea is


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mentioned here only because we were already discussing convergence in this section and it ties into the last topic that we want to discuss in this section. In the previous section after we’d introduced the idea of an infinite series we commented on the fact that we shouldn’t think of an infinite series as an infinite sum despite the fact that the notation we use for infinite series seems to imply that it is an infinite sum. It’s now time to briefly discuss this. First, we need to introduce the idea of a rearrangement. A rearrangement of a series is exactly what it might sound like, it is the same series with the terms rearranged into a different order. For example, consider the following the infinite series.

1 2 3 4 5 6 71

nn

a a a a a a a a∞

=

= + + + + + + +∑ L

A rearrangement of this series is,

2 1 3 14 5 9 41

nn

a a a a a a a a∞

=

= + + + + + + +∑ L

The issue we need to discuss here is that for some series each of these arrangements of terms can have a different values despite the fact that they are using exactly the same terms. Here is an example of this. It can be shown that,

( ) 1

1

1 1 1 1 1 1 1 11 ln 22 3 4 5 6 7 8

n

nn

∞ +

=

−= − + − + − + − + =∑ L (1)

Since this series converges we know that if we multiply it by a constant c its value will also be multiplied by c. So, let’s multiply this by 1

2 to get,

1 1 1 1 1 1 1 1 1 ln 22 4 6 8 10 12 14 16 2

− + − + − + − + =L (2)

Now, let’s add in a zero between each term as follows.

1 1 1 1 1 1 10 0 0 0 0 0 0 ln 22 4 6 8 10 12 2

+ + − + + + − + + + − + + =L (3)

Note that this won’t change the value of the series because the partial sums for this series will be the partial sums for the (2) except that each term will be repeated. Repeating terms in a series will not affect its limit however and so both (2) and (3) will be the same. We know that if two series converge we can add them by adding term by term and so add (1) and (3) to get,


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1 1 1 1 1 31 ln 23 2 5 7 4 2

+ − + + − + =L (4)

Now, notice that the terms of (4) are simply the terms of (1) rearranged so that each negative term comes after two positive terms. The values however are definitely different despite the fact that the terms are the same. Note as well that this is not one of those “tricks” that you see occasionally where you get a contradictory result because of a hard to spot math/logic error. This is a very real result and we’ve not made any logic mistakes/errors. Here is a nice set of facts that govern this idea of when a rearrangement will lead to a different value of a series. Facts Given the series na∑ ,

1. If na∑ is absolutely convergent and its value is s then any rearrangement of na∑ will also have a value of s.

2. If na∑ is conditionally convergent and r is any real number then there is a

rearrangement of na∑ whose value will be r. Again, we do not have the tools in hand yet to determine if a series is absolutely convergent and so don’t worry about this at this point. This is here just to make sure that you understand that we have to be very careful in thinking of an infinite series as an infinite sum. There are times when we can (i.e. the series is absolutely convergent) and there are times when we can’t (i.e. the series is conditionally convergent). As a final note, the fact above tells us that the series,

( ) 1

1

1 n

nn

∞ +

=

−∑

must be conditionally convergent since two rearrangements gave two separate values of this series. Eventually it will be very simple to show that this series conditionally convergent.


Math 140


Series – Special Series In this section we are going to take a brief look at three special series. Actually, special may not be the correct term. All three have been named which makes them special in some way, however the main reason that we’re going to look at two of them in this section is that they are the only types of series that we’ll be looking at for which we will be able to get actual values for the series. The third type is divergent and so won’t have a value to worry about. In general, determining the value of a series is very difficult and outside of these two kinds of series that we’ll look at in this section we will not be determining the value of series in this chapter. So, let’s get started. Geometric Series A geometric series is any series that can be written in the form,

1

1

n

nar

∞−

=∑

or, with an index shift the geometric series will often be written as,

0

n

nar

∞

=∑

These are identical series and will have identical values, provided they converge of course. If we start with the first form it can be shown that the partial sums are,

( )11 1 1

n n

n

a r a arsr r r

−= = −

− − −

The series will converge provided the partial sums form a convergent sequence, so let’s take the limit of the partial sums.

lim lim1 1

lim lim1 1

lim1 1

n

nn n

n

n n

n

n

a arsr r

a arr r

a a rr r

→∞ →∞

→∞ →∞

→∞

= − − −

= −− −

= −− −

Now, from Theorem 3 from the Sequences section we know that the limit above will exist and be finite provided 1 1r− < ≤ . However, note that we can’t let 1r = since this will give division by zero. Therefore, this will exist and be finite provided 1 1r− < < and in this case the limit is zero and so we get,


Math 140


lim1nn

asr→∞

=−

Therefore, a geometric series will converge if 1 1r− < < , which is usually written 1r < , its

value is,

1

1 0 1n n

n n

aar arr

∞ ∞−

= =

= =−∑ ∑

Note that in using this formula we’ll need to make sure that we are in the correct form. In other words, if the series starts at 0n = then the exponent on the r must be n. Likewise if the series starts at 1n = then the exponent on the r must be 1n − . Example 1 Determine if the following series converge or diverge. If they converge give the value of the series.

(a) 2 1

19 4n n

n

∞− + +

=∑ [Solution]

(b) ( )3

10

45

n

nn

∞

−=

−∑ [Solution]

Solution

(a) 2 1

19 4n n

n

∞− + +

=∑

This series doesn’t really look like a geometric series. However, notice that both parts of the series term are numbers raised to a power. This means that it can be put into the form of a geometric series. We will just need to decide which form is the correct form. Since the series starts at 1n = we will want the exponents on the numbers to be 1n − . It will be fairly easy to get this into the correct form. Let’s first rewrite things slightly. One of the n’s in the exponent has a negative in front of it and that can’t be there in the geometric form. So, let’s first get rid of that.

( )1

22 1 12

1 1 1

49 4 9 49

nnn n n

nn n n

+∞ ∞ ∞− −− + + +

−= = =

= =∑ ∑ ∑

Now let’s get the correct exponent on each of the numbers. This can be done using simple exponent properties.

1 1 2

2 12 1 1

1 1 1

4 4 49 49 9 9

n nn n

n nn n n

+ −∞ ∞ ∞− + +

− − −= = =

= =∑ ∑ ∑

Now, rewrite the term a little.


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( )11

2 11

1 1 1

4 49 4 16 9 1449 9

nnn n

nn n n

−−∞ ∞ ∞− + +

−= = =

= =

∑ ∑ ∑

So, this is a geometric series with 144a = and 4

9 1r = < . Therefore, since 1r < we know the

series will converge and its value will be,

( )2 1

1

144 9 12969 4 1444 5 519

n n

n

∞− + +

=

= = =−

∑


(b) ( )3

10

45

n

nn

∞

−=

−∑

Again, this doesn’t look like a geometric series, but it can be put into the correct form. In this case the series starts at 0n = so we’ll need the exponents to be n on the terms. Note that this means we’re going to need to rewrite the exponent on the numerator a little

( ) ( )( ) ( )33

1 10 0 0 0

44 64 645 55 5 5 5 5

nn n n

n n nn n n n

∞ ∞ ∞ ∞

− −= = = =

−− − − = = =

∑ ∑ ∑ ∑

So, we’ve got it into the correct form and we can see that 5a = and 64

5r = − . Also note that

1r ≥ and so this series diverges. [Return to Problems]

Back in the Series – Basics section we talked about stripping out terms from a series, but didn’t really provide any examples of how this idea could be used in practice. We can now do some examples. Example 2 Use the results from the previous example to determine the value of the following series.

(a) 2 1

09 4n n

n

∞− + +

=∑ [Solution]

(b) 2 1

39 4n n

n

∞− + +

=∑ [Solution]

Solution

(a) 2 1

09 4n n

n

∞− + +

=∑

In this case we could just acknowledge that this is a geometric series that starts at 0n = and so


Math 140


we could put it into the correct form and be done with it. However, this does provide us with a nice example of how to use the idea of stripping out terms to our advantage. Let’s notice that if we strip out the first term from this series we arrive at,

2 1 2 1 2 1 2 1

0 1 19 4 9 4 9 4 324 9 4n n n n n n

n n n

∞ ∞ ∞− + + − + + − + +

= = =

= + = +∑ ∑ ∑

From the previous example we know the value of the new series that arises here and so the value of the series in this example is,

2 1

0

1296 29169 4 3245 5

n n

n

∞− + +

=

= + =∑


(b) 2 1

39 4n n

n

∞− + +

=∑

In this case we can’t strip out terms from the given series to arrive at the series used in the previous example. However, we can start with the series used in the previous example and strip terms out of it to get the series in this example. So, let’s do that. We will strip out the first two terms from the series we looked at in the previous example.

2 1 1 2 0 3 2 1 2 1

1 3 39 4 9 4 9 4 9 4 208 9 4n n n n n n

n n n

∞ ∞ ∞− + + − + + − + +

= = =

= + + = +∑ ∑ ∑

We can now use the value of the series from the previous example to get the value of this series.

2 1 2 1

3 1

1296 2569 4 9 4 208 2085 5

n n n n

n n

∞ ∞− + + − + +

= =

= − = − =∑ ∑

[Return to Problems] Notice that we didn’t discuss the convergence of either of the series in the above example. Here’s why. Consider the following series written in two separate ways (i.e. we stripped out a couple of terms from it).

0 1 20 3

n nn n

a a a a a∞ ∞

= =

= + + +∑ ∑

Let’s suppose that we know 3

nn

a∞

=∑ is a convergent series. This means that it’s got a finite value

and adding three finite terms onto this will not change that fact. So the value of 0

nn

a∞

=∑ is also

finite and so is convergent.


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Likewise, suppose that 0

nn

a∞

=∑ is convergent. In this case if we subtract three finite values from

this value we will remain finite and arrive at the value of 3

nn

a∞

=∑ . This is now a finite value and

so this series will also be convergent. In other words, if we have two series and they differ only by the presence, or absence, of a finite number of finite terms they will either both be convergent or they will both be divergent. The difference of a few terms one way or the other will not change the convergence of a series. This is an important idea and we will use it several times in the following sections to simplify some of the tests that we’ll be looking at. Telescoping Series It’s now time to look at the second of the three series in this section. In this portion we are going to look at a series that is called a telescoping series. The name in this case comes from what happens with the partial sums and is best shown in an example. Example 3 Determine if the following series converges or diverges. If it converges find its value.

20

13 2n n n

∞

= + +∑

Solution We first need the partial sums for this series.

20

13 2

n

ni

si i=

=+ +∑

Now, let’s notice that we can use partial fractions on the series term to get,

( ) ( )2

1 1 1 13 2 2 1 1 2i i i i i i

= = −+ + + + + +

I’ll leave the details of the partial fractions to you. By now you should be fairly adept at this since we spent a fair amount of time doing partial fractions back in the Integration Techniques chapter. If you need a refresher you should go back and review that section. So, what does this do for us? Well, let’s start writing out the terms of the general partial sum for this series using the partial fraction form.


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0

1 11 2

1 11 2

n

ni

si i=

= − + +

= −

∑12

+

13

−13

+

14

−1n

+ +

L 1

1n−

+1

1n

+ +

12

112

n

n

− +

= −+

Notice that every term except the first and last term canceled out. This is the origin of the name telescoping series. This also means that we can determine the convergence of this series by taking the limit of the partial sums.

1lim lim 1 12nn n

sn→∞ →∞

= − = +

The sequence of partial sums is convergent and so the series is convergent and has a value of

20

1 13 2n n n

∞

=

=+ +∑

In telescoping series be careful to not assume that successive terms will be the ones that cancel. Consider the following example. Example 4 Determine if the following series converges or diverges. If it converges find its value.

21

14 3n n n

∞

= + +∑

Solution As with the last example we’ll leave the partial fractions details to you to verify. The partial sums are,

1 12 2

1 1

1 1 11 3 2 1 3

1 1 12 2 4

n n

ni i

si i i i= =

= − = − + + + +

= −

∑ ∑

1 13 5

+ −

14

+

16

−1n

+ +

L 1 1

2 1n n

− + + +

13

1 1 1 1 12 2 3 2 3

n

n n

− +

= + − − + +

In this case instead of successive terms canceling a term will cancel with a term that is farther down the list. The end result this time is two initial and two final terms are left. Notice as well that in order to help with the work a little we factored the 1

2 out of the series.


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The limit of the partial sums is,

1 5 1 1 5lim lim2 6 2 3 12nn n

sn n→∞ →∞

= − − = + +

So, this series is convergent (because the partial sums form a convergent sequence) and its value is,

21

1 54 3 12n n n

∞

=

=+ +∑

Note that it’s not always obvious if a series is telescoping or not until you try to get the partial sums and then see if they are in fact telescoping. There is no test that will tell us that we’ve got a telescoping series right off the bat. Also note that just because you can do partial fractions on a series term does not mean that the series will be a telescoping series. The following series, for example, is not a telescoping series despite the fact that we can partial fraction the series terms.

21 1

3 2 1 13 2 1 2n n

nn n n n

∞ ∞

= =

+ = + + + + + ∑ ∑

In order for a series to be a telescoping we must get terms to cancel and all of these terms are positive and so none will cancel. Next, we need to go back and address an issue that was first raised in the previous section. In that section we stated that the sum or difference of convergent series was also convergent and that the presence of a multiplicative constant would not affect the convergence of a series. Now that we have a few more series in hand let’s work a quick example showing that. Example 5 Determine the value of the following series.

2 12

1

4 9 44 3

n n

n n n

∞− + +

=

− + + ∑

Solution To get the value of this series all we need to do is rewrite it and then use the previous results.

2 1 2 12 2

1 1 1

2 12

1 1

4 49 4 9 44 3 4 3

14 9 44 3

5 1296412 5

386315

n n n n

n n n

n n

n n

n n n n

n n

∞ ∞ ∞− + + − + +

= = =

∞ ∞− + +

= =

− = − + + + +

= −+ +

= −

= −

∑ ∑ ∑

∑ ∑

We didn’t discuss the convergence of this series because it was the sum of two convergent series


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and that guaranteed that the original series would also be convergent. Harmonic Series This is the third and final series that we’re going to look at in this section. Here is the harmonic series.

1

1n n

∞

=∑

The harmonic series is divergent and we’ll need to wait until the next section to show that. This series is here because it’s got a name and so I wanted to put it here with the other two named series that we looked at in this section. We’re also going to use the harmonic series to illustrate a couple of ideas about divergent series that we’ve already discussed for convergent series. We’ll do that with the following example. Example 6 Show that each of the following series are divergent.

(a) 1

5n n

∞

=∑

(b) 4

1n n

∞

=∑

Solution

(a) 1

5n n

∞

=∑

To see that this series is divergent all we need to do is use the fact that we can factor a constant out of a series as follows,

1 1

5 15n nn n

∞ ∞

= =

=∑ ∑

Now, 1

1n n

∞

=∑ is divergent and so five times this will still not be a finite number and so the series

has to be divergent. In other words, if we multiply a divergent series by a constant it will still be divergent.

(b) 4

1n n

∞

=∑

In this case we’ll start with the harmonic series and strip out the first three terms.

1 4 4 1

1 1 1 1 1 1 1112 3 6n n n nn n n n

∞ ∞ ∞ ∞

= = = =

= + + + ⇒ = −

∑ ∑ ∑ ∑

In this case we are subtracting a finite number from a divergent series. This subtraction will not change the divergence of the series. We will either have infinity minus a finite number, which is still infinity, or a series with no value minus a finite number, which will still have no value.


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Therefore, this series is divergent. Just like with convergent series, adding/subtracting a finite number from a divergent series is not going to change the fact the convergence of the series. So, some general rules about the convergence/divergence of a series are now in order. Multiplying a series by a constant will not change the convergence/divergence of the series and adding or subtracting a constant from a series will not change the convergence/divergence of the series. These are nice ideas to keep in mind.


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Vectors

Introduction This is a fairly short chapter. We will be taking a brief look at vectors and some of their properties. We will need some of this material in the next chapter and those of you heading on towards Calculus III will use a fair amount of this there as well. Here is a list of topics in this chapter. Vectors – The Basics – In this section we will introduce some of the basic concepts about vectors. Vector Arithmetic – Here we will give the basic arithmetic operations for vectors. Dot Product – We will discuss the dot product in this section as well as an application or two. Cross Product – In this section we’ll discuss the cross product and see a quick application.


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Vectors – The Basics Let’s start this section off with a quick discussion on what vectors are used for. Vectors are used to represent quantities that have both a magnitude and a direction. Good examples of quantities that can be represented by vectors are force and velocity. Both of these have a direction and a magnitude. Let’s consider force for a second. A force of say 5 Newtons that is applied in a particular direction can be applied at any point in space. In other words, the point where we apply the force does not change the force itself. Forces are independent of the point of application. To define a force all we need to know is the magnitude of the force and the direction that the force is applied in. The same idea holds more generally with vectors. Vectors only impart magnitude and direction. They don’t impart any information about where the quantity is applied. This is an important idea to always remember in the study of vectors. In a graphical sense vectors are represented by directed line segments. The length of the line segment is the magnitude of the vector and the direction of the line segment is the direction of the vector. However, because vectors don’t impart any information about where the quantity is applied any directed line segment with the same length and direction will represent the same vector. Consider the sketch below.

Each of the directed line segments in the sketch represents the same vector. In each case the vector starts at a specific point then moves 2 units to the left and 5 units up. The notation that we’ll use for this vector is, 2,5v = −

r and each of the directed line segments in the sketch are called representations of the vector.


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Be careful to distinguish vector notation, 2,5− , from the notation we use to represent

coordinates of points, ( )2,5− . The vector denotes a magnitude and a direction of a quantity

while the point denotes a location in space. So don’t mix the notations up! A representation of the vector 1 2,v a a=

r in two dimensional space is any directed line segment,

ABuuur

, from the point ( ),A x y= to the point ( )1 2,B x a y a= + + . Likewise a representation of

the vector 1 2 3, ,v a a a=r

in three dimensional space is any directed line segment, ABuuur

, from the

point ( ), ,A x y z= to the point ( )1 2 3, ,B x a y a z a= + + + .

Note that there is very little difference between the two dimensional and three dimensional formulas above. To get from the three dimensional formula to the two dimensional formula all we did is take out the third component/coordinate. Because of this most of the formulas here are given only in their three dimensional version. If we need them in their two dimensional form we can easily modify the three dimensional form. There is one representation of a vector that is special in some way. The representation of the vector 1 2 3, ,v a a a=

r that starts at the point ( )0,0,0A = and ends at the point ( )1 2 3, ,B a a a=

is called the position vector of the point ( )1 2 3, ,a a a . So, when we talk about position vectors

we are specifying the initial and final point of the vector. Position vectors are useful if we ever need to represent a point as a vector. As we’ll see there are times in which we definitely are going to want to represent points as vectors. In fact, we’re going to run into topics that can only be done if we represent points as vectors. Next we need to discuss briefly how to generate a vector given the initial and final points of the representation. Given the two points ( )1 2 3, ,A a a a= and ( )1 2 3, ,B b b b= the vector with the

representation ABuuur

is, 1 1 2 2 3 3, ,v b a b a b a= − − −

r Note that we have to be very careful with direction here. The vector above is the vector that starts

at A and ends at B. The vector that starts at B and ends at A, i.e. with representation BAuuur

is, 1 1 2 2 3 3, ,w a b a b a b= − − −

r These two vectors are different and so we do need to always pay attention to what point is the starting point and what point is the ending point. When determining the vector between two points we always subtract the initial point from the terminal point.


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Example 1 Give the vector for each of the following. (a) The vector from ( )2, 7,0− to ( )1, 3, 5− − .

(b) The vector from ( )1, 3, 5− − to ( )2, 7,0− .

(c) The position vector for ( )90,4− Solution (a) Remember that to construct this vector we subtract coordinates of the starting point from the ending point.

( )1 2, 3 7 , 5 0 1, 4, 5− − − − − − = − − (b) Same thing here.

( ) ( )2 1, 7 3 ,0 5 1, 4,5− − − − − − = − Notice that the only difference between the first two is the signs are all opposite. This difference is important as it is this difference that tells us that the two vectors point in opposite directions. (c) Not much to this one other than acknowledging that the position vector of a point is nothing more than a vector with the points coordinates as its components. 90, 4− We now need to start discussing some of the basic concepts that we will run into on occasion. Magnitude The magnitude, or length, of the vector 1 2 3, ,v a a a=

r is given by,

2 2 21 2 3v a a a= + +

Example 2 Determine the magnitude of each of the following vectors.

(a) 3, 5,10a = −r

(b) 1 2,5 5

u = −r

(c) 0,0w =r

(d) 1,0,0i =r

Solution There isn’t too much to these other than plug into the formula.

(a) 9 25 100 134a = + + =r

(b) 1 4 1 15 5

u = + = =r

(c) 0 0 0w = + =r

(d) 1 0 0 1i = + + =r


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We also have the following fact about the magnitude.

If 0 then 0a a= =rr r

This should make sense. Because we square all the components the only way we can get zero out of the formula was for the components to be zero in the first place. Unit Vector Any vector with magnitude of 1, i.e. 1u =

r, is called a unit vector.

Example 3 Which of the vectors from Example 2 are unit vectors? Solution Both the second and fourth vectors had a length of 1 and so they are the only unit vectors from the first example. Zero Vector The vector 0,0w =

r that we saw in the first example is called a zero vector since its

components are all zero. Zero vectors are often denoted by 0r

. Be careful to distinguish 0 (the

number) from 0r

(the vector). The number 0 denote the origin in space, while the vector 0r

denotes a vector that has no magnitude or direction. Standard Basis Vectors The fourth vector from the second example, 1,0,0i =

r, is called a standard basis vector. In

three dimensional space there are three standard basis vectors,

1,0,0 0,1,0 0,0,1i j k= = =rr r

In two dimensional space there are two standard basis vectors, 1,0 0,1i j= =

r r

Note that standard basis vectors are also unit vectors. Warning We are pretty much done with this section however, before proceeding to the next section we should point out that vectors are not restricted to two dimensional or three dimensional space. Vectors can exist in general n-dimensional space. The general notation for a n-dimensional vector is,


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1 2 3, , , , nv a a a a=r …

and each of the ai’s are called components of the vector. Because we will be working almost exclusively with two and three dimensional vectors in this course most of the formulas will be given for the two and/or three dimensional cases. However, most of the concepts/formulas will work with general vectors and the formulas are easily (and naturally) modified for general n-dimensional vectors. Also, because it is easier to visualize things in two dimensions most of the figures related to vectors will be two dimensional figures. So, we need to be careful to not get too locked into the two or three dimensional cases from our discussions in this chapter. We will be working in these dimensions either because it’s easier to visualize the situation or because physical restrictions of the problems will enforce a dimension upon us.


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Vector Arithmetic In this section we need to have a brief discussion of vector arithmetic. We’ll start with addition of two vectors. So, given the vectors 1 2 3, ,a a a a=

r and

1 2 3, ,b b b b=r

the addition of the two vectors is given by the following formula.

1 1 2 2 3 3, ,a b a b a b a b+ = + + +rr

The following figure gives the geometric interpretation of the addition of two vectors.

This is sometimes called the parallelogram law or triangle law. Computationally, subtraction is very similar. Given the vectors 1 2 3, ,a a a a=

r and

1 2 3, ,b b b b=r

the difference of the two vectors is given by,

1 1 2 2 3 3, ,a b a b a b a b− = − − −rr

Here is the geometric interpretation of the difference of two vectors.


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It is a little harder to see this geometric interpretation. To help see this let’s instead think of

subtraction as the addition of ar and b−r

. First, as we’ll see in a bit b−r

is the same vector as br

with opposite signs on all the components. In other words, b−r

goes in the opposite direction as

br

. Here is the vector set up for ( )a b+ −rr

.

As we can see from this figure we can move the vector representing ( )a b+ −rr

to the position

we’ve got in the first figure showing the difference of the two vectors. Note that we can’t add or subtract two vectors unless they a have the same number of components. If they don’t have the same number of components then addition and subtraction can’t be done. The next arithmetic operation that we want to look at is scalar multiplication. Given the vector

1 2 3, ,a a a a=r

and any number c the scalar multiplication is,

1 2 3, ,ca ca ca ca=r


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So, we multiply all the components by the constant c. To see the geometric interpretation of scalar multiplication let’s take a look at an example. Example 1 For the vector 2, 4a =

r compute 3ar , 1

2 ar and 2a−r

. Graph all four vectors on the same axis system. Solution Here are the three scalar multiplications.

13 6,12 1, 2 2 4, 82

a a a= = − = − −r r r

Here is the graph for each of these vectors.

In the previous example we can see that if c is positive all scalar multiplication will do is stretch (if 1c > ) or shrink (if 1c < ) the original vector, but it won’t change the direction. Likewise, if c is negative scalar multiplication will switch the direction so that the vector will point in exactly the opposite direction and it will again stretch or shrink the magnitude of the vector depending upon the size of c. There are several nice applications of scalar multiplication that we should now take a look at. The first is parallel vectors. This is a concept that we will see quite a bit over the next couple of sections. Two vectors are parallel if they have the same direction or are in exactly opposite directions. Now, recall again the geometric interpretation of scalar multiplication. When we performed scalar multiplication we generated new vectors that were parallel to the original vectors (and each other for that matter).

So, let’s suppose that ar and br

are parallel vectors. If they are parallel then there must be a number c so that,

a cb=rr

So, two vectors are parallel if one is a scalar multiple of the other.


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Example 2 Determine if the sets of vectors are parallel or not. (a) 2, 4,1 , 6,12, 3a b= − = − −

rr

(b) 4,10 , 2, 9a b= = −rr

Solution

(a) These two vectors are parallel since 3b a= −r r

(b) These two vectors aren’t parallel. This can be seen by noticing that ( )1

24 2= and yet

( )1210 5 9= ≠ − . In other words we can’t make ar be a scalar multiple of b

r.

The next application is best seen in an example. Example 3 Find a unit vector that points in the same direction as 5,2,1w = −

r.

Solution Okay, what we’re asking for is a new parallel vector (points in the same direction) that happens to be a unit vector. We can do this with a scalar multiplication since all scalar multiplication does is change the length of the original vector (along with possibly flipping the direction to the opposite direction). Here’s what we’ll do. First let’s determine the magnitude of wr .

25 4 1 30w = + + =r

Now, let’s form the following new vector,

1 1 5 2 15,2,1 , ,30 30 30 30

u ww

= = − = −r r

r

The claim is that this is a unit vector. That’s easy enough to check

25 4 1 30 130 30 30 30

u = + + = =r

This vector also points in the same direction as wr since it is only a scalar multiple of wr and we used a positive multiple.

So, in general, given a vector wr , wuw

=rrr will be a unit vector that points in the same direction

as wr .


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Standard Basis Vectors Revisited In the previous section we introduced the idea of standard basis vectors without really discussing why they were important. We can now do that. Let’s start with the vector

1 2 3, ,a a a a=r

We can use the addition of vectors to break this up as follows,

1 2 3

1 2 3

, ,

,0,0 0, ,0 0,0,

a a a a

a a a

=

= + +

r

Using scalar multiplication we can further rewrite the vector as,

1 2 3

1 2 3

,0,0 0, ,0 0,0,

1,0,0 0,1,0 0,0,1

a a a a

a a a

= + +

= + +

r

Finally, notice that these three new vectors are simply the three standard basis vectors for three dimensional space.

1 2 3 1 2 3, ,a a a a i a j a k= + +rr r

So, we can take any vector and write it in terms of the standard basis vectors. From this point on we will use the two notations interchangeably so make sure that you can deal with both notations.

Example 4 If 3, 9,1a = −r

and 8w i k= − +rrr

compute 2 3a w−r r

. Solution In order to do the problem we’ll convert to one notation and then perform the indicated operations.

2 3 2 3, 9,1 3 1,0,8

6, 18,2 3,0, 24

9, 18, 22

a w− = − − −

= − − −

= − −

r r

We will leave this section with some basic properties of vector arithmetic. Properties If vr , wr and ur are vectors (each with the same number of components) and a and b are two numbers then we have the following properties.

( ) ( )

( ) ( )0 1

v w w v u v w u v w

v v v va v w av aw a b v av bv

+ = + + + = + +

+ = =

+ = + + = +

r r r r r r r r r rrr r r r

r r r r r r r


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The proofs of these are pretty much just “computation” proofs so we’ll prove one of them and leave the others to you to prove. Proof of ( )a v w av aw+ = +

r r r r

We’ll start with the two vectors, 1 2, , , nv v v v=r … and 1 2, , , nw w w w=

r … and yes we did

mean for these to each have n components. The theorem works for general vectors so we may as well do the proof for general vectors. Now, as noted above this is pretty much just a “computational” proof. What that means is that we’ll compute the left side and then do some basic arithmetic on the result to show that we can make the left side look like the right side. Here is the work.

( ) ( )

( ) ( ) ( )

1 2 1 2

1 1 2 2

1 1 2 2

1 1 2 2

1 2 1 2

1 2 1 2

, , , , , ,

, , ,

, , ,

, , ,

, , , , , ,

, , , , , ,

n n

n n

n n

n n

n n

n n

a v w a v v v w w w

a v w v w v w

a v w a v w a v w

av aw av aw av aw

av av av aw aw aw

a v v v a w w w av aw

+ = +

= + + +

= + + +

= + + +

= +

= + = +

r r … …

…

…

…

… …r r… …


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Dot Product The next topic for discussion is that of the dot product. Let’s jump right into the definition of the

dot product. Given the two vectors 1 2 3, ,a a a a=r

and 1 2 3, ,b b b b=r

the dot product is,

1 1 2 2 3 3a b a b a b a b= + +rri (1)

Sometimes the dot product is called the scalar product. The dot product is also an example of an inner product and so on occasion you may hear it called an inner product. Example 1 Compute the dot product for each of the following.

(a) 5 8 , 2v i j w i j= − = +r r r rr r

(b) 0,3, 7 , 2,3,1a b= − =rr

Solution Not much to do with these other than use the formula. (a) 5 16 11v w = − = −

r ri

(b) 0 9 7 2a b = + − =rri

Here are some properties of the dot product. Properties

( ) ( ) ( ) ( )

2

0 0

If 0 then 0

u v w u v u w cv w v cw c v w

v w w v v

v v v v v v

+ = + = =

= =

= = =

r r r r r r r r r r r r ri i i i i irr r r r ri i i

rr r r r r ri i

The proofs of these properties are mostly “computational” proofs and so we’re only going to do a couple of them and leave the rest to you to prove. Proof of ( )u v w u v u w+ = +

r r r r r r ri i i

We’ll start with the three vectors, 1 2, , , nu u u u=r … , 1 2, , , nv v v v=

r … and

1 2, , , nw w w w=r … and yes we did mean for these to each have n components. The theorem

works for general vectors so we may as well do the proof for general vectors. Now, as noted above this is pretty much just a “computational” proof. What that means is that we’ll compute the left side and then do some basic arithmetic on the result to show that we can make the left side look like the right side. Here is the work.


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( ) ( )

( ) ( ) ( )

1 2 1 2 1 2

1 2 1 1 2 2

1 1 1 2 2 2

1 1 1 1 2 2 2 2

1 1 2 2 1 1 2 2

1 2 1 2 1 2 1

, , , , , , , , ,

, , , , , ,

, , ,

, , ,

, , , , , ,

, , , , , , , , ,

n n n

n n n

n n n

n n n n

n n n n

n n n

u v w u u u v v v w w w

u u u v w v w v w

u v w u v w u v w

u v u w u v u w u v u w

u v u v u v u w u w u w

u u u v v v u u u w

+ = +

= + + +

= + + +

= + + +

= +

= +

r r ri … i … …

… i …

…

…

… …

… i … … i 2, , , nw wu v u w= +

…r r r ri i

Proof of : If 0v v =r ri then 0v =

rr

This is a pretty simple proof. Let’s start with 1 2, , , nv v v v=r … and compute the dot product.

1 2 1 2

2 2 21 2

, , , , , ,

0

n n

n

v v v v v v v v

v v v

=

= + + +=

r ri … i …

L

Now, since we know 2 0iv ≥ for all i then the only way for this sum to be zero is to in fact have

2 0iv = . This in turn however means that we must have 0iv = and so we must have had 0v =rr

.

There is also a nice geometric interpretation to the dot product. First suppose that θ is the angle

between ar and br

such that 0 θ π≤ ≤ as shown in the image below.

We can then have the following theorem.


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Theorem

cosa b a b θ=r rr ri (2)

Proof Let’s give a modified version of the sketch above.

The three vectors above form the triangle AOB and note that the length of each side is nothing more than the magnitude of the vector forming that side. The Law of Cosines tells us that,

2 22 2 cosa b a b a b θ− = + −

r r rr r r

Also using the properties of dot products we can write the left side as,

( ) ( )2

22 2

a b a b a b

a a a b b a b b

a a b b

− = − −

= − − +

= − +

r r rr r rir r r rr r r ri i i i

r rr ri

Our original equation is then,


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2 22

2 22 2

2 cos

2 2 cos

2 2 cos

cos

a b a b a b

a a b b a b a b

a b a b

a b a b

θ

θ

θ

θ

− = + −

− + = + −

− = −

=

r r rr r r

r r r rr r r rir rr rir rr ri

The formula from this theorem is often used not to compute a dot product but instead to find the angle between two vectors. Note as well that while the sketch of the two vectors in the proof is for two dimensional vectors the theorem is valid for vectors of any dimension (as long as they have the same dimension of course). Let’s see an example of this.

Example 2 Determine the angle between 3, 4, 1a = − −r

and 0,5, 2b =r

. Solution We will need the dot product as well as the magnitudes of each vector.

22 26 29a b a b= − = =r rr ri

The angle is then,

( )1

22cos 0.801192726 29

cos 0.8011927 2.5 radians=143.24 degrees

a ba b

θ

θ −

−= = = −

= − =

rri rr

The dot product gives us a very nice method for determining if two vectors are perpendicular and it will give another method for determining when two vectors are parallel. Note as well that often we will use the term orthogonal in place of perpendicular. Now, if two vectors are orthogonal then we know that the angle between them is 90 degrees. From (2) this tells us that if two vectors are orthogonal then,

0a b =rri


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Likewise, if two vectors are parallel then the angle between them is either 0 degrees (pointing in the same direction) or 180 degrees (pointing in the opposite direction). Once again using (2) this would mean that one of the following would have to be true.

( ) ( )0 OR 180a b a b a b a bθ θ= = ° = − = °r r r rr r r ri i

Example 3 Determine if the following vectors are parallel, orthogonal, or neither.

(a) 6, 2, 1 , 2,5, 2a b= − − =rr

(b) 1 12 ,2 4

u i j v i j= − = − +r r r rr r

Solution (a) First get the dot product to see if they are orthogonal.

12 10 2 0a b = − − =rri

The two vectors are orthogonal. (b) Again, let’s get the dot product first.

1 514 4

u v = − − = −r ri

So, they aren’t orthogonal. Let’s get the magnitudes and see if they are parallel.

5 5516 4

u v= = =r r

Now, notice that,

5 554 4

u v u v

= − = − = −

r r r ri

So, the two vectors are parallel. There are several nice applications of the dot product as well that we should look at. Projections The best way to understand projections is to see a couple of sketches. So, given two vectors ar

and br

we want to determine the projection of br

onto ar . The projection is denoted by proja brr

. Here are a couple of sketches illustrating the projection.


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So, to get the projection of br

onto ar we drop straight down from the end of br

until we hit (and form a right angle) with the line that is parallel to ar . The projection is then the vector that is parallel to ar , starts at the same point both of the original vectors started at and ends where the dashed line hits the line parallel to ar .

There is an nice formula for finding the projection of br

onto ar . Here it is,

2projaa bb aa

=r

rrr i rr

Note that we also need to be very careful with notation here. The projection of ar onto br

is given by

2projba ba bb

=r

rr rirr

We can see that this will be a totally different vector. This vector is parallel to br

, while proja brr

is parallel to ar . So, be careful with notation and make sure you are finding the correct projection. Here’s an example.

Example 4 Determine the projection of 2,1, 1b = −r

onto 1,0, 2a = −r

. Solution We need the dot product and the magnitude of ar .

24 5a b a= =rr ri

The projection is then,


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2proj

4 1,0, 254 8,0,5 5

aa bb aa

=

= −

= −

r

rrr i rr

For comparison purposes let’s do it the other way around as well.

Example 5 Determine the projection of 1,0, 2a = −r

onto 2,1, 1b = −r

. Solution

We need the dot product and the magnitude of br

.

2

4 6a b b= =r rri

The projection is then,

2proj

4 2,1, 164 2 2, ,3 3 3

ba ba bb

=

= −

= −

r

rr rirr

As we can see from the previous two examples the two projections are different so be careful. Direction Cosines This application of the dot product requires that we be in three dimensional space unlike all the other applications we’ve looked at to this point. Let’s start with a vector, ar , in three dimensional space. This vector will form angles with the x-axis (α ), the y-axis (β ), and the z-axis (γ ). These angles are called direction angles and the cosines of these angles are called direction cosines. Here is a sketch of a vector and the direction angles.


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The formulas for the direction cosines are,

31 2cos cos cos aa aa i a j a ka a a a a a

α β γ= = = = = =rr rr r ri i i

r r r r r r

where ir

, jr

and kr

are the standard basis vectors.

Let’s verify the first dot product above. We’ll leave the rest to you to verify. 1 2 3 1, , 1,0,0a i a a a a= =

rri i Here are a couple of nice facts about the direction cosines.

1. The vector cos ,cos ,cosu α β γ=r

is a unit vector.

2. 2 2 2cos cos cos 1α β γ+ + = 3. cos ,cos ,cosa a α β γ=

r r

Let’s do a quick example involving direction cosines. Example 6 Determine the direction cosines and direction angles for 2,1, 4a = −

r.

Solution We will need the magnitude of the vector.

4 1 16 21a = + + =r


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The direction cosines and angles are then,

2cos 1.119 radians 64.123 degrees211cos 1.351 radians 77.396 degrees214cos 2.632 radians 150.794 degrees21

α α

β β

γ γ

= = =

= = =

−= = =


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Cross Product In this final section of this chapter we will look at the cross product of two vectors. We should note that the cross product requires both of the vectors to be three dimensional vectors. Also, before getting into how to compute these we should point out a major difference between dot products and cross products. The result of a dot product is a number and the result of a cross product is a vector! Be careful not to confuse the two.

So, let’s start with the two vectors 1 2 3, ,a a a a=r

and 1 2 3, ,b b b b=r

then the cross product is

given by the formula,

2 3 3 2 3 1 1 3 1 2 2 1, ,a b a b a b a b a b a b a b× = − − −rr

This is not an easy formula to remember. There are two ways to derive this formula. Both of them use the fact that the cross product is really the determinant of a 3x3 matrix. If you don’t know what this is that is don’t worry about it. You don’t need to know anything about matrices or determinants to use either of the methods. The notation for the determinant is as follows,

1 2 3

1 2 3

i j ka b a a a

b b b× =

rr rrr

The first row is the standard basis vectors and must appear in the order given here. The second

row is the components of ar and the third row is the components of br

. Now, let’s take a look at the different methods for getting the formula. The first method uses the Method of Cofactors. If you don’t know the method of cofactors that is fine, the result is all that we need. Here is the formula.

2 3 1 3 1 2

2 3 1 3 1 2

a a a a a aa b i j k

b b b b b b× = − +

r rr rr

where,

a b

ad bcc d

= −

This formula is not as difficult to remember as it might at first appear to be. First, the terms alternate in sign and notice that the 2x2 is missing the column below the standard basis vector that multiplies it as well as the row of standard basis vectors.


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The second method is slightly easier; however, many textbooks don’t cover this method as it will only work on 3x3 determinants. This method says to take the determinant as listed above and then copy the first two columns onto the end as shown below.

1 2 3 1 2

1 2 3 1 2

i j k i ja b a a a a a

b b b b b× =

rr r r rrr

We now have three diagonals that move from left to right and three diagonals that move from right to left. We multiply along each diagonal and add those that move from left to right and subtract those that move from right to left. This is best seen in an example. We’ll also use this example to illustrate a fact about cross products.

Example 1 If 2,1, 1a = −r

and 3,4,1b = −r

compute each of the following.

(a) a b×rr

(b) b a×

r r

Solution (a) Here is the computation for this one.

( )( ) ( )( ) ( )( ) ( ) ( ) ( )( ) ( )( )

2 1 1 2 13 4 1 3 4

1 1 1 3 2 4 2 1 1 4 1 3

5 11

i j k i ja b

i j k j i k

i j k

× = −− −

= + − − + − − − − −

= + +

rr r r rrr

r rr r r r

rr r

(b) And here is the computation for this one.

( ) ( ) ( )( ) ( ) ( ) ( )( ) ( ) ( ) ( ) ( )

3 4 1 3 42 1 1 2 1

4 1 1 2 3 1 3 1 1 1 4 2

5 11

i j k i jb a

i j k j i k

i j k

× = − −−

= − + + − − − − − −

= − − −

rr r r rr r

r rr r r r

rr r

Notice that switching the order of the vectors in the cross product simply changed all the signs in the result. Note as well that this means that the two cross products will point in exactly opposite directions since they only differ by a sign. We’ll formalize up this fact shortly when we list several facts. There is also a geometric interpretation of the cross product. First we will let θ be the angle

between the two vectors ar and br

and assume that 0 θ π≤ ≤ , then we have the following fact,


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sina b a b θ× =r rr r (1)

and the following figure.

There should be a natural question at this point. How did we know that the cross product pointed in the direction that we’ve given it here? First, as this figure, implies the cross product is orthogonal to both of the original vectors. This will always be the case with one exception that we’ll get to in a second. Second, we knew that it pointed in the upward direction (in this case) by the “right hand rule”.

This says that if we take our right hand, start at ar and rotate our fingers towards br

our thumb

will point in the direction of the cross product. Therefore, if we’d sketched in b a×r r

above we would have gotten a vector in the downward direction. Example 2 A plane is defined by any three points that are in the plane. If a plane contains the points ( )1,0,0P = , ( )1,1,1Q = and ( )2, 1,3R = − find a vector that is orthogonal to the plane. Solution The one way that we know to get an orthogonal vector is to take a cross product. So, if we could find two vectors that we knew were in the plane and took the cross product of these two vectors we know that the cross product would be orthogonal to both the vectors. However, since both the vectors are in the plane the cross product would then also be orthogonal to the plane. So, we need two vectors that are in the plane. This is where the points come into the problem. Since all three points lie in the plane any vector between them must also be in the plane. There are many ways to get two vectors between these points. We will use the following two,

1 1,1 0,1 0 0,1,1

2 1, 1 0,3 0 1, 1,3

PQ

PR

= − − − =

= − − − − = −

uuur

uuur

The cross product of these two vectors will be orthogonal to the plane. So, let’s find the cross


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product.

0 1 1 0 11 1 3 1 1

4

i j k i jPQ PR

i j k

× =− −

= + −

rr r r ruuur uuur

rr r

So, the vector 4i j k+ −rr r

will be orthogonal to the plane containing the three points. Now, let’s address the one time where the cross product will not be orthogonal to the original

vectors. If the two vectors, ar and br

, are parallel then the angle between them is either 0 or 180 degrees. From (1) this implies that,

0a b× =rr

From a fact about the magnitude we saw in the first section we know that this implies

0a b× =r rr

In other words, it won’t be orthogonal to the original vectors since we have the zero vector. This does give us another test for parallel vectors however. Fact

If 0a b× =r rr

then ar and br

will be parallel vectors. Let’s also formalize up the fact about the cross product being orthogonal to the original vectors. Fact

Provided 0a b× ≠r rr

then a b×rr

is orthogonal to both ar and br

. Here are some nice properties about the cross product. Properties If ur , vr and wr are vectors and c is a number then,

( ) ( ) ( )

( ) ( ) ( )

( )1 2 3

1 2 3

1 2 3

u v v u cu v u cv c u v

u v w u v u w u v w u v w

u u uu v w v v v

w w w

× = − × × = × = ×

× + = × + × × = ×

× =

r r r r r r r r r r

r r r r r r r r r r r r ri i

r r ri

The determinant in the last fact is computed in the same way that the cross product is computed. We will see an example of this computation shortly.


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There are a couple of geometric applications to the cross product as well. Suppose we have three

vectors ar , br

and cr and we form the three dimensional figure shown below.

The area of the parallelogram (two dimensional front of this object) is given by,

Area a b= ×rr

and the volume of the parallelpiped (the whole three dimensional object) is given by,

( )Volume a b c= ×rr ri

Note that the absolute value bars are required since the quantity could be negative and volume isn’t negative. We can use this volume fact to determine if three vectors lie in the same plane or not. If three vectors lie in the same plane then the volume of the parallelpiped will be zero.

Example 3 Determine if the three vectors 1, 4, 7a = −r

, 2, 1, 4b = −r

and 0, 9,18c = −r

lie in the same plane or not. Solution So, as we noted prior to this example all we need to do is compute the volume of the parallelpiped formed by these three vectors. If the volume is zero they lie in the same plane and if the volume isn’t zero they don’t lie in the same plane.


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( )

( )( )( ) ( )( ) ( ) ( ) ( )( )( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )

1 4 7 1 42 1 4 2 10 9 18 0 9

1 1 18 4 4 0 7 2 9

4 2 18 1 4 9 7 1 018 126 144 36

0

a b c−

× = − −− −

= − + + − − −

− − − − −

= − + − +=

rr ri

So, the volume is zero and so they lie in the same plane.


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Three Dimensional Space

Introduction In this chapter we will start taking a more detailed look at three dimensional space (3-D space or

3¡ ). This is a very important topic for Calculus III since a good portion of Calculus III is done in three (or higher) dimensional space. We will be looking at the equations of graphs in 3-D space as well as vector valued functions and how we do calculus with them. We will also be taking a look at a couple of new coordinate systems for 3-D space. This is the only chapter that exists in two places in my notes. When I originally wrote these notes all of these topics were covered in Calculus II however, we have since moved several of them into Calculus III. So, rather than split the chapter up I have kept it in the Calculus II notes and also put a copy in the Calculus III notes. Many of the sections not covered in Calculus III will be used on occasion there anyway and so they serve as a quick reference for when we need them. Here is a list of topics in this chapter. The 3-D Coordinate System – We will introduce the concepts and notation for the three dimensional coordinate system in this section. Equations of Lines – In this section we will develop the various forms for the equation of lines in three dimensional space. Equations of Planes – Here we will develop the equation of a plane. Quadric Surfaces – In this section we will be looking at some examples of quadric surfaces.


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The 3-D Coordinate System We’ll start the chapter off with a fairly short discussion introducing the 3-D coordinate system and the conventions that we’ll be using. We will also take a brief look at how the different coordinate systems can change the graph of an equation. Let’s first get some basic notation out of the way. The 3-D coordinate system is often denoted by

3¡ . Likewise the 2-D coordinate system is often denoted by 2¡ and the 1-D coordinate system is denoted by ¡ . Also, as you might have guessed then a general n dimensional coordinate system is often denoted by n¡ . Next, let’s take a quick look at the basic coordinate system.

This is the standard placement of the axes in this class. It is assumed that only the positive directions are shown by the axes. If we need the negative axis for any reason we will put them in as needed. Also note the various points on this sketch. The point P is the general point sitting out in 3-D space. If we start at P and drop straight down until we reach a z-coordinate of zero we arrive at the point Q. We say that Q sits in the xy-plane. The xy-plane corresponds to all the points which have a zero z-coordinate. We can also start at P and move in the other two directions as shown to get points in the xz-plane (this is S with a y-coordinate of zero) and the yz-plane (this is R with an x-coordinate of zero). Collectively, the xy, xz, and yz-planes are sometimes called the coordinate planes. In the remainder of this class you will need to be able to deal with the various coordinate planes so make sure that you can. Also, the point Q is often referred to as the projection of P in the xy-plane. Likewise, R is the projection of P in the yz-plane and S is the projection of P in the xz-plane.


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Many of the formulas that you are used to working with in 2¡ have natural extensions in 3¡ . For instance the distance between two points in 2¡ is given by,

( ) ( ) ( )2 21 2 2 1 2 1,d P P x x y y= − + −

While the distance between any two points in 3¡ is given by,

( ) ( ) ( ) ( )2 2 21 2 2 1 2 1 2 1,d P P x x y y z z= − + − + −

Likewise, the general equation for a circle with center ( ),h k and radius r is given by,

( ) ( )2 2 2x h y k r− + − =

and the general equation for a sphere with center ( ), ,h k l and radius r is given by,

( ) ( ) ( )2 2 2 2x h y k z l r− + − + − = With that said we do need to be careful about just translating everything we know about 2¡ into

3¡ and assuming that it will work the same way. A good example of this is in graphing to some extent. Consider the following example. Example 1 Graph 3x = in ¡ , 2¡ and 3¡ . Solution In ¡ we have a single coordinate system and so 3x = is a point in a 1-D coordinate system. In 2¡ the equation 3x = tells us to graph all the points that are in the form ( )3, y . This is a

vertical line in a 2-D coordinate system. In 3¡ the equation 3x = tells us to graph all the points that are in the form ( )3, ,y z . If you go

back and look at the coordinate plane points this is very similar to the coordinates for the yz-plane except this time we have 3x = instead of 0x = . So, in a 3-D coordinate system this is a plane that will be parallel to the yz-plane and pass through the x-axis at 3x = . Here is the graph of 3x = in ¡ .

Here is the graph of 3x = in 2¡ .


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Finally, here is the graph of 3x = in 3¡ . Note that we’ve presented this graph in two different styles. On the left we’ve got the traditional axis system and we’re used to seeing and on the right we’ve put the graph in a box. Both views can be convenient on occasion to help with perspective and so we’ll often do this with 3D graphs and sketches.

Note that at this point we can now write down the equations for each of the coordinate planes as well using this idea.

0 plane0 plane0 plane

z xyy xzx yz

= −= −= −

Let’s take a look at a slightly more general example.


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Example 2 Graph 2 3y x= − in 2¡ and 3¡ . Solution Of course we had to throw out ¡ for this example since there are two variables which means that we can’t be in a 1-D space. In 2¡ this is a line with slope 2 and a y intercept of -3. However, in 3¡ this is not necessarily a line. Because we have not specified a value of z we are forced to let z take any value. This means that at any particular value of z we will get a copy of this line. So, the graph is then a vertical plane that lies over the line given by 2 3y x= − in the xy-plane. Here is the graph in 2¡ .

here is the graph in 3¡ .


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Notice that if we look to where the plane intersects the xy-plane we will get the graph of the line in 2¡ as noted in the above graph by the red line through the plane. Let’s take a look at one more example of the difference between graphs in the different coordinate systems. Example 3 Graph 2 2 4x y+ = in 2¡ and 3¡ . Solution As with the previous example this won’t have a 1-D graph since there are two variables. In 2¡ this is a circle centered at the origin with radius 2. In 3¡ however, as with the previous example, this may or may not be a circle. Since we have not specified z in any way we must assume that z can take on any value. In other words, at any value of z this equation must be satisfied and so at any value z we have a circle of radius 2 centered on the z-axis. This means that we have a cylinder of radius 2 centered on the z-axis. Here are the graphs for this example.


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Notice that again, if we look to where the cylinder intersects the xy-plane we will again get the circle from 2¡ . We need to be careful with the last two examples. It would be tempting to take the results of these and say that we can’t graph lines or circles in 3¡ and yet that doesn’t really make sense. There is no reason for there to not be graphs lines or circles in 3¡ . Let’s think about the example of the circle. To graph a circle in 3¡ we would need to do something like 2 2 4x y+ = at 5z = . This would be a circle of radius 2 centered on the z-axis at the level of 5z = . So, as long as we specify a z we will get a circle and not a cylinder. We will see an easier way to specify circles in a later section. We could do the same thing with the line from the second example. However, we will be looking at line in more generality in the next section and so we’ll see a better way to deal with lines in 3¡ there. The point of the examples in this section is to make sure that we are being careful with graphing equations and making sure that we always remember which coordinate system that we are in.


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Another quick point to make here is that, as we’ve seen in the above examples, many graphs of equations in 3¡ are surfaces. That doesn’t mean that we can’t graph curves in 3¡ . We can and will graph curves in 3¡ as well as we’ll see later in this chapter.


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Equations of Lines

In this section we need to take a look at the equation of a line in 3¡ . As we saw in the previous section the equation y mx b= + does not describe a line in 3¡ , instead it describes a plane. This doesn’t mean however that we can’t write down an equation for a line in 3-D space. We’re just going to need a new way of writing down the equation of a curve. So, before we get into the equations of lines we first need to briefly look at vector functions. We’re going to take a more in depth look at vector functions later. At this point all that we need to worry about is notational issues and how they can be used to give the equation of a curve. The best way to get an idea of what a vector function is and what its graph looks like is to look at an example. So, consider the following vector function. ( ) ,1r t t=

r A vector function is a function that takes one or more variables, one in this case, and returns a vector. Note as well that a vector function can be a function of two or more variables. However, in those cases the graph may no longer be a curve in space. The vector that the function gives can be a vector in whatever dimension we need it to be. In the example above it returns a vector in 2¡ . When we get to the real subject of this section, equations of lines, we’ll be using a vector function that returns a vector in 3¡ Now, we want to determine the graph of the vector function above. In order to find the graph of our function we’ll think of the vector that the vector function returns as a position vector for points on the graph. Recall that a position vector, say ,v a b=

r, is a vector that starts at the

origin and ends at the point ( ),a b .

So, to get the graph of a vector function all we need to do is plug in some values of the variable and then plot the point that corresponds to each position vector we get out of the function and play connect the dots. Here are some evaluations for our example. ( ) ( ) ( ) ( )3 3,1 1 1,1 2 2,1 5 5,1r r r r− = − − = − = =

r r r r So, each of these are position vectors representing points on the graph of our vector function. The points, ( ) ( ) ( ) ( )3,1 1,1 2,1 5,1− − are all points that lie on the graph of our vector function. If we do some more evaluations and plot all the points we get the following sketch.


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In this sketch we’ve included the position vector (in gray and dashed) for several evaluations as well as the t (above each point) we used for each evaluation. It looks like, in this case the graph of the vector equation is in fact the line 1y = . Here’s another quick example. Here is the graph of ( ) 6cos ,3sinr t t t=

r.

In this case we get an ellipse. It is important to not come away from this section with the idea that vector functions only graph out lines. We’ll be looking at lines in this section, but the graphs of vector function do not have to be lines as the example above shows. We’ll leave this brief discussion of vector function with another way to think of the graph of a vector function. Imagine that a pencil/pen is attached to the end of the position vector and as we increase the variable the resulting position vector moves and as it moves the pencil/pen on the end sketches out the curve for the vector function.


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Okay, we now need to move into the actual topic of this section. We want to write down the equation of a line in 3¡ and as suggested by the work above we will need a vector function to do this. To see how we’re going to do this let’s think about what we need to write down the equation of a line in 2¡ . In two dimensions we need the slope (m) and a point that was on the line in order to write down the equation. In 3¡ that is still all that we need except in this case the “slope” won’t be a simple number as it was in two dimensions. In this case we will need to acknowledge that a line can have a three dimensional slope. So, we need something that will allow us to describe a direction that is potentially in three dimensions. We already have a quantity that will do this for us. Vectors give directions and can be three dimensional objects. So, let’s start with the following information. Suppose that we know a point that is on the line,

( )0 0 0 0, ,P x y z= , and that , ,v a b c=r

is some vector that is parallel to the line. Note, in all

likelihood, vr will not be on the line itself. We only need vr to be parallel to the line. Finally, let

( ), ,P x y z= be any point on the line.

Now, since our “slope” is a vector let’s also represent the two points on the line as vectors. We’ll

do this with position vectors. So, let 0rur

and rr be the position vectors for P0 and P respectively.

Also, for no apparent reason, let’s define ar to be the vector with representation 0P Puuur

. We now have the following sketch with all these points and vectors on it.

Now, we’ve shown the parallel vector, vr , as a position vector but it doesn’t need to be a position vector. It can be anywhere, a position vector, on the line or off the line, it just needs to be parallel to the line.


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Next, notice that we can write rr as follows,

0r r a= +urr r

If you’re not sure about this go back and check out the sketch for vector addition in the vector arithmetic section. Now, notice that the vectors ar and vr are parallel. Therefore there is a number, t, such that a t v=

r r We now have,

0 0 0 0, , , ,r r t v x y z t a b c= + = +urr r

This is called the vector form of the equation of a line. The only part of this equation that is not known is the t. Notice that t vr will be a vector that lies along the line and it tells us how far from the original point that we should move. If t is positive we move away from the original point in the direction of vr (right in our sketch) and if t is negative we move away from the original point in the opposite direction of vr (left in our sketch). As t varies over all possible values we will completely cover the line. The following sketch shows this dependence on t of our sketch.

There are several other forms of the equation of a line. To get the first alternate form let’s start with the vector form and do a slight rewrite.

0 0 0

0 0 0

, , , ,

, , , ,

r x y z t a b c

x y z x ta y tb z tc

= +

= + + +

r


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The only way for two vectors to be equal is for the components to be equal. In other words,

0

0

0

x x tay y tbz z tc

= += += +

This set of equations is called the parametric form of the equation of a line. Notice as well that this is really nothing more than an extension of the parametric equations we’ve seen previously. The only difference is that we are now working in three dimensions instead of two dimensions. To get a point on the line all we do is pick a t and plug into either form of the line. In the vector form of the line we get a position vector for the point and in the parametric form we get the actual coordinates of the point. There is one more form of the line that we want to look at. If we assume that a, b, and c are all non-zero numbers we can solve each of the equations in the parametric form of the line for t. We can then set all of them equal to each other since t will be the same number in each. Doing this gives the following,

0 0 0x x y y z za b c− − −

= =

This is called the symmetric equations of the line. If one of a, b, or c does happen to be zero we can still write down the symmetric equations. To see this let’s suppose that 0b = . In this case t will not exist in the parametric equation for y and so we will only solve the parametric equations for x and z for t. We then set those equal and acknowledge the parametric equation for y as follows,

0 00

x x z z y ya c− −

= =

Let’s take a look at an example. Example 1 Write down the equation of the line that passes through the points ( )2, 1,3− and

( )1, 4, 3− . Write down all three forms of the equation of the line. Solution To do this we need the vector vr that will be parallel to the line. This can be any vector as long as it’s parallel to the line. In general, vr won’t lie on the line itself. However, in this case it will. All we need to do is let vr be the vector that starts at the second point and ends at the first point. Since these two points are on the line the vector between them will also lie on the line and will


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hence be parallel to the line. So, 1, 5,6v = −

r Note that the order of the points was chosen to reduce the number of minus signs in the vector. We could just have easily gone the other way. Once we’ve got vr there really isn’t anything else to do. To use the vector form we’ll need a point on the line. We’ve got two and so we can use either one. We’ll use the first point. Here is the vector form of the line. 2, 1,3 1, 5,6 2 , 1 5 ,3 6r t t t t= − + − = + − − +

r Once we have this equation the other two forms follow. Here are the parametric equations of the line.

2

1 53 6

x ty tz t

= += − −= +

Here is the symmetric form.

2 1 31 5 6

x y z− + −= =

−

Example 2 Determine if the line that passes through the point ( )0, 3,8− and is parallel to the line given by 10 3x t= + , 12y t= and 3z t= − − passes through the xz-plane. If it does give the coordinates of that point. Solution To answer this we will first need to write down the equation of the line. We know a point on the line and just need a parallel vector. We know that the new line must be parallel to the line given by the parametric equations in the problem statement. That means that any vector that is parallel to the given line must also be parallel to the new line. Now recall that in the parametric form of the line the numbers multiplied by t are the components of the vector that is parallel to the line. Therefore, the vector, 3,12, 1v = −

r is parallel to the given line and so must also be parallel to the new line. The equation of new line is then, 0, 3,8 3,12, 1 3 , 3 12 ,8r t t t t= − + − = − + −

r If this line passes through the xz-plane then we know that the y-coordinate of that point must be zero. So, let’s set the y component of the equation equal to zero and see if we can solve for t. If


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we can, this will give the value of t for which the point will pass through the xz-plane. 13 12 04

t t− + = ⇒ =

So, the line does pass through the xz-plane. To get the complete coordinates of the point all we

need to do is plug 14

t = into any of the equations. We’ll use the vector form.

1 1 1 3 313 , 3 12 ,8 ,0,4 4 4 4 4

r = − + − =

r

Recall that this vector is the position vector for the point on the line and so the coordinates of the

point where the line will pass through the xz-plane are 3 31,0,4 4

.


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Equations of Planes In the first section of this chapter we saw a couple of equations of planes. However, none of those equations had three variables in them and were really extensions of graphs that we could look at in two dimensions. We would like a more general equation for planes. So, let’s start by assuming that we know a point that is on the plane, ( )0 0 0 0, ,P x y z= . Let’s also

suppose that we have a vector that is orthogonal (perpendicular) to the plane, , ,n a b c=r

. This

vector is called the normal vector. Now, assume that ( ), ,P x y z= is any point in the plane.

Finally, since we are going to be working with vectors initially we’ll let 0rur

and rr be the position vectors for P0 and P respectively. Here is a sketch of all these vectors.

Notice that we added in the vector 0r r−urr

which will lie completely in the plane. Also notice that we put the normal vector on the plane, but there is actually no reason to expect this to be the case. We put it here to illustrate the point. It is completely possible that the normal vector does not touch the plane in any way. Now, because nr is orthogonal to the plane, it’s also orthogonal to any vector that lies in the

plane. In particular it’s orthogonal to 0r r−urr

. Recall from the Dot Product section that two orthogonal vectors will have a dot product of zero. In other words,

( )0 00n r r n r n r− = ⇒ =ur urr r r r ri i i


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This is called the vector equation of the plane. A slightly more useful form of the equations is as follows. Start with the first form of the vector equation and write down a vector for the difference.

( )0 0 0

0 0 0

, , , , , , 0

, , , , 0

a b c x y z x y z

a b c x x y y z z

− =

− − − =

i

i

Now, actually compute the dot product to get, ( ) ( ) ( )0 0 0 0a x x b y y c z z− + − + − =

This is called the scalar equation of plane. Often this will be written as, ax by cz d+ + = where 0 0 0d ax by cz= + + . This second form is often how we are given equations of planes. Notice that if we are given the equation of a plane in this form we can quickly get a normal vector for the plane. A normal vector is, , ,n a b c=

r Let’s work a couple of examples. Example 1 Determine the equation of the plane that contains the points ( )1, 2,0P = − ,

( )3,1,4Q = and ( )0, 1,2R = − . Solution In order to write down the equation of plane we need a point (we’ve got three so we’re cool there) and a normal vector. We need to find a normal vector. Recall however, that we saw how to do this in the Cross Product section. We can form the following two vectors from the given points.

2,3, 4 1,1,2PQ PR= = −uuur uuur

These two vectors will lie completely in the plane since we formed them from points that were in the plane. Notice as well that there are many possible vectors to use here, we just chose two of the possibilities. Now, we know that the cross product of two vectors will be orthogonal to both of these vectors. Since both of these are in the plane any vector that is orthogonal to both of these will also be orthogonal to the plane. Therefore, we can use the cross product as the normal vector.


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2 3 4 2 3 2 8 51 1 2 1 1

i j k i jn PQ PR i j k= × = = − +

− −

rr r r ruuur uuur rr rr

The equation of the plane is then,

( ) ( ) ( )2 1 8 2 5 0 0

2 8 5 18x y z

x y z− − + + − =

− + =

We used P for the point, but could have used any of the three points. Example 2 Determine if the plane given by 2 10x z− + = and the line given by

5, 2 ,10 4r t t= − +r

are orthogonal, parallel or neither. Solution This is not as difficult a problem as it may at first appear to be. We can pick off a vector that is normal to the plane. This is 1,0, 2n = −

r. We can also get a vector that is parallel to the line.

This is 0, 1, 4v = − .

Now, if these two vectors are parallel then the line and the plane will be orthogonal. If you think about it this makes some sense. If nr and vr are parallel, then vr is orthogonal to the plane, but vr is also parallel to the line. So, if the two vectors are parallel the line and plane will be orthogonal. Let’s check this.

1 0 2 1 0 2 4 00 1 4 0 1

i j k i jn v i j k× = − − = + + ≠

− −

rr r r rr rr rr r

So, the vectors aren’t parallel and so the plane and the line are not orthogonal. Now, let’s check to see if the plane and line are parallel. If the line is parallel to the plane then any vector parallel to the line will be orthogonal to the normal vector of the plane. In other words, if nr and vr are orthogonal then the line and the plane will be parallel. Let’s check this.

0 0 8 8 0n v = + + = ≠r ri

The two vectors aren’t orthogonal and so the line and plane aren’t parallel. So, the line and the plane are neither orthogonal nor parallel.


Math 140


Quadric Surfaces

In the previous two sections we’ve looked at lines and planes in three dimensions (or 3¡ ) and while these are used quite heavily at times in a Calculus class there are many other surfaces that are also used fairly regularly and so we need to take a look at those. In this section we are going to be looking at quadric surfaces. Quadric surfaces are the graphs of any equation that can be put into the general form 2 2 2 0Ax By Cz Dxy Exz Fyz Gx Hy Iz J+ + + + + + + + + = where A, … , J are constants. There is no way that we can possibly list all of them, but there are some standard equations so here is a list of some of the more common quadric surfaces. Ellipsoid Here is the general equation of an ellipsoid.

2 2 2

2 2 2 1x y za b c

+ + =

Here is a sketch of a typical ellipsoid.

If a b c= = then we will have a sphere. Notice that we only gave the equation for the ellipsoid that has been centered on the origin. Clearly ellipsoids don’t have to be centered on the origin. However, in order to make the discussion in this section a little easier we have chosen to concentrate on surfaces that are “centered” on the origin in one way or another. Cone Here is the general equation of a cone.

2 2 2

2 2 2

x y za b c

+ =


Math 140


Here is a sketch of a typical cone.

Note that this is the equation of a cone that will open along the z-axis. To get the equation of a cone that opens along one of the other axes all we need to do is make a slight modification of the equation. This will be the case for the rest of the surfaces that we’ll be looking at in this section as well. In the case of a cone the variable that sits by itself on one side of the equal sign will determine the axis that the cone opens up along. For instance, a cone that opens up along the x-axis will have the equation,

2 2 2

2 2 2

y z xb c a

+ =

For most of the following surfaces we will not give the other possible formulas. We will however acknowledge how each formula needs to be changed to get a change of orientation for the surface. Cylinder Here is the general equation of a cylinder.

2 2

2 2 1x ya b

+ =

This is a cylinder whose cross section is an ellipse. If a b= we have a cylinder whose cross section is a circle. We’ll be dealing with those kinds of cylinders more than the general form so the equation of a cylinder with a circular cross section is, 2 2 2x y r+ =


Math 140


Here is a sketch of typical cylinder with an ellipse cross section.

The cylinder will be centered on the axis corresponding to the variable that does not appear in the equation. Be careful to not confuse this with a circle. In two dimensions it is a circle, but in three dimensions it is a cylinder. Hyperboloid of One Sheet Here is the equation of a hyperboloid of one sheet.

2 2 2

2 2 2 1x y za b c

+ − =

Here is a sketch of a typical hyperboloid of one sheet.


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The variable with the negative in front of it will give the axis along which the graph is centered. Hyperboloid of Two Sheets Here is the equation of a hyperboloid of two sheets.

2 2 2

2 2 2 1x y za b c

− − + =

Here is a sketch of a typical hyperboloid of two sheets.

The variable with the positive in front of it will give the axis along which the graph is centered.


Math 140


Notice that the only difference between the hyperboloid of one sheet and the hyperboloid of two sheets is the signs in front of the variables. They are exactly the opposite signs. Elliptic Paraboloid Here is the equation of an elliptic paraboloid.

2 2

2 2

x y za b c

+ =

As with cylinders this has a cross section of an ellipse and if a b= it will have a cross section of a circle. When we deal with these we’ll generally be dealing with the kind that have a circle for a cross section. Here is a sketch of a typical elliptic paraboloid.

In this case the variable that isn’t squared determines the axis upon which the paraboloid opens up. Also, the sign of c will determine the direction that the paraboloid opens. If c is positive then it opens up and if c is negative then it opens down. Hyperbolic Paraboloid Here is the equation of a hyperbolic paraboloid.

2 2

2 2

x y za b c

− =

Here is a sketch of a typical hyperbolic paraboloid.


Math 140


These graphs are vaguely saddle shaped and as with the elliptic paraoloid the sign of c will determine the direction in which the surface “opens up”. The graph above is shown for c positive. With the both of the types of paraboloids discussed above the surface can be easily moved up or down by adding/subtracting a constant from the left side. For instance 2 2 6z x y= − − + is an elliptic paraboloid that opens downward (be careful, the “-” is on the x and y instead of the z) and starts at 6z = instead of 0z = . Here is a couple of quick sketches of this surface.


Math 140


Note that we’ve given two forms of the sketch here. The sketch on the right has the standard set of axes but it is difficult to see the numbers on the axis. The sketch on the left has been “boxed” and this makes it easier to see the numbers to give a sense of perspective to the sketch. In most sketches that actually involve numbers on the axis system we will give both sketches to help get a feel for what the sketch looks like.


Math 140 by Paul... · Math 140 © 2007 Paul Dawkins 3 Integrals Introduction In this chapter we...

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