Expansion of Binomials

(x+y)n

The expansion of a binomial follows a predictable pattern

Learn the pattern and you can expand any binomial

What are we doing?

Expanding binomials of the form (x+y)n

Looking for patterns in the expansion of binomials

Developing a method for expanding binomials

Why are we doing this?

Topic in Intermediate and College Algebra

Necessary in Calculus if not for the Chain Rule

What have we learned before that will help?

Distributive property of real numbers

Multiplying polynomials

How will I know if I have learned this?

You will be able to expand any binomial of the form (x+y)n without the laborious task of successive multiplications of (x+y)

(x+y)1

(x+y)1=x+y

What is the degree of the expansion? How many terms are in the expansion? What is the exponent of x in the first term? What is the exponent of y in the first term? What is the sum of the exponents in the first term?

(x+y)1

(x+y)1=x+y

What is the exponent of x in the second term? What is the exponent of y in the second term? What is the sum of the exponents in the second term? What is the coefficient of the first term? What is the coefficient of the second term?

(x+y)2

(x+y)(x+y)

(x+y)2=(x+y)(x+y)

x + y x + y

Write the first expression twice for the two terms in the second expression

(x+y)2=(x+y)(x+y)

x + y x + y

x x y y

Place each term of the second expression below

(x+y)2=(x+y)(x+y)

x + y x + y

x x y y

x2 xy xy y2

Multiply down the columns, then combine like terms

(x+y)2

x2+2xy+y2

(x+y)2

(x+y)2=x2+2xy+y2

What is the degree of the expansion? How many terms are in the expansion? What is the exponent of x in the first term? What is the exponent of y in the first term? What is the sum of the exponents in the first term?

(x+y)2

(x+y)2=x2+2xy+y2

What is the exponent of x in the second term? What is the exponent of y in the second term? What is the sum of the exponents in the second term? What is the exponent of x in the third term? What is the exponent of y in the third term? What is the sum of the exponents in the third term?

(x+y)2

(x+y)2=x2+2xy+y2

How do the exponents of x change from left to right? How do the exponents of y change from left to right? What is the coefficient of the first term? What is the coefficient of the second term? What is the coefficient of the third term?

(x+y)3

(x+y)3=(x+y)2(x+y)

(x+y)3=(x2+2xy+y2)(x+y)

x2 + 2xy + y2 x2 + 2xy + y2

Write the first expression twice for the two terms in the second expression

(x+y)3=(x2+2xy+y2)(x+y)

x2 + 2xy + y2 x2 + 2xy + y2

x x x y y y

Place each term of the second expression below

(x+y)3=(x2+2xy+y2)(x+y)

x2 + 2xy + y2 x2 + 2xy + y2

x x x y y y

x3 2x2y xy2 x2y 2xy2 y3

Multiply down the columns, then combine like terms

(x+y)3

(x+y)3=x3+3x2y+3xy2+y3

(x+y)3

What is the degree of the expansion? How many terms are in the expansion? What is the exponent of x in the first term? What is the exponent of y in the first term? What is the sum of the exponents in the first term? What is the exponent of x in the second term? What is the exponent of y in the second term? What is the sum of the exponents in the second term?

(x+y)3=x3+3x2y+3xy2+y3

(x+y)3

What is the exponent of x in the third term? What is the exponent of y in the third term? What is the sum of the exponents in the third term? What is the exponent of x in the fourth term? What is the exponent of y in the fourth term? What is the sum of the exponents in the fourth term?

(x+y)3=x3+3x2y+3xy2+y3

(x+y)3

How do the exponents of x change from left to right? How do the exponents of y change from left to right? What is the coefficient of the first term? What is the coefficient of the second term? What is the coefficient of the third term?

(x+y)3=x3+3x2y+3xy2+y3

(x+y)4

(x+y)4=(x+y)3(x+y)

(x+y)4=(x3+3x2y+3xy2+y3)(x+y)

x3 + 3x2y + 3xy2 + y3 x3 + 3x2y + 3xy2 + y3

Write the first expression twice for the two terms in the second expression

(x+y)4=(x3+3x2y+3xy2+y3)(x+y)

x3 + 3x2y + 3xy2 + y3 x3 + 3x2y + 3xy2 + y3

x x x x y y y y

Place each term of the second expression below

(x+y)4=(x3+3x2y+3xy2+y3)(x+y)

x3 + 3x2y + 3xy2 + y3 x3 + 3x2y + 3xy2 + y3

x x x x y y y y

x4 3x3y 3x2y2 xy3 x3y 3x2y2 3xy3 y4

Multiply down the columns, then combine like terms

(x+y)4

(x+y)4=x4+4x3y+6x2y2+4xy3+y4

(x+y)4

What is the degree of the expansion? How many terms are in the expansion? What is the exponent of x in the first term? What is the exponent of y in the first term? What is the sum of the exponents in the first term? What is the exponent of x in the second term? What is the exponent of y in the second term? What is the sum of the exponents in the second term?

(x+y)4=x4+4x3y+6x2y2+4xy3+y4

(x+y)4

What is the exponent of x in the third term? What is the exponent of y in the third term? What is the sum of the exponents in the third term? What is the exponent of x in the fourth term? What is the exponent of y in the fourth term? What is the sum of the exponents in the fourth term? What is the exponent of x in the fifth term? What is the exponent of y in the fifth term? What is the sum of the exponents in the fifth term?

(x+y)4=x4+4x3y+6x2y2+4xy3+y4

(x+y)4

How do the exponents of x change from left to right? How do the exponents of y change from left to right? What is the coefficient of the first term? What is the coefficient of the second term? What is the coefficient of the third term? What is the coefficient of the fourth term? What is the coefficient of the fifth term?

(x+y)4=x4+4x3y+6x2y2+4xy3+y4

Pattern of exponents degrees 1 to 4

n degree # terms

sum of exponents

1 1 2 1

2 2 3 2

3 3 4 3

4 4 5 4

degree of expansion of binomial = n

number of terms in expansion = n+1

sum of exponents in each term = n

exponent of x decreases from n to 0

exponent of y increases from 0 to n

Pattern of coefficients degrees 1 to 4

degree coefficients

1 1 1

2 1 2 1

3 1 3 3 1

4 1 4 6 4 1

What is the pattern from row to row?

Coefficients of 5th degree expansion

degree coefficients

4 1 4 6 4 1

5 1 5 10 10 5 1

This pattern of coefficients is called Pascal’s Triangle

It can be extended to find the coefficients of any degree expansion of a binomial

(x+y)5

What is the degree of the expansion? How many terms are in the expansion?

(x+y)5

xy + xy + xy + xy + xy + xy What is the exponent of x in the first term? What is the exponent of y in the first term? What is the exponent of x in the second term? What is the exponent of y in the second term? What is the exponent of x in the third term? What is the exponent of y in the third term?

(x+y)5

xy + xy + xy + xy + xy + xy What is the exponent of x in the fourth term? What is the exponent of y in the fourth term? What is the exponent of x in the fifth term? What is the exponent of y in the fifth term? What is the exponent of x in the sixth term? What is the exponent of y in the sixth term?

(x+y)5

Based on the pattern for binomial coefficients: What is the binomial coefficient of the first term? What is the binomial coefficient of the second term? What is the binomial coefficient of the third term? What is the binomial coefficient of the fourth term? What is the binomial coefficient of the fifth term? What is the binomial coefficient of the sixth term?

(x+y)5=

x5+5x4y+10x3y2+10x2y3+5xy4+y5

Would you want to build Pascal’s Triangle for (x+y)99?

You could, but it would be a large triangle.

Is there a short cut?Yes, indeed there is!

Factorials

n!

Unary operatorSymbol !Multiplication of all numbers

from n down to 1

0!=1n!=n·(n-1)!=n·(n-1)·(n-2)!n!/(n-2)!=n·(n-1)·(n-2)!/(n-2)!=n·(n-1)

(nr) means n choose r =n!/(n-r)!r!

(x+y)n

Binomial Theorem

For r = 0 to n

The (r+1)th term isn!/(n-r)!r!x(n-r)yr

(x+y)7

n=7r=00+1=1st term7!/(7-0)!0!x(7-0)y0=

7!/7!x7y0=x7

(x+y)7

n=7r=11+1=2nd term7!/(7-1)!1!x(7-1)y1=

7!/6!x6y1=7·6!/6!x6y1=7x6y

(x+y)7

n=7r=22+1=3rd term7!/(7-2)!2!x(7-2)y2=

7!/5!2!x5y2=7·6·5!/5!2!x5y2=7·6/2x5y2=7·3x5y2=21x5y2

(x+y)7

n=7r=33+1=4th term7!/(7-3)!3!x(7-3)y3=

7!/4!3!x4y3=7·6·5·4!/4!3!x4y3=7·6·5/3·2·1x4y3=7·5x4y3=35x4y3

(x+y)7

n=7r=44+1=5th term7!/(7-4)!4!x(7-4)y4=

7!/3!4!x3y4=7·6·5·4!/3!4!x3y4=7·6·5/3·2·1x3y4=7·5x3y4=35x3y4

(x+y)7

n=7r=55+1=6th term7!/(7-5)!5!x(7-5)y5=

7!/2!5!x2y5=7·6·5!/2!5!x2y5=7·6/2·1x2y5=7·3x2y5=21x2y5

(x+y)7

n=7r=66+1=7th term7!/(7-6)!6!x(7-6)y6=

7!/1!6!x1y6=7·6!/6!x1y6=7xy6

(x+y)7

n=7r=77+1=8th term7!/(7-7)!7!x(7-7)y7=

7!/0!7!x0y7=7!/7!x0y7=y7

(x+y)7

x7+7x6y+21x5y2+35x4y3+

35x3y4+21x2y5+7xy6+y7

What about x + a number

(x+6)4=

x4+4x3(6)1+6x2(6)2+4x(6)3+(6)4=

x4+24x3+216x2+864x+1296

Knowing the binomial theorem

Can help you factor polynomials

8x3+36x2+54x+27

(x+y)3=x3+3x2y+3xy2+y3

Is the first term a cube? 8x3=(2x)3

Is the fourth term a cube? 27=(3)3

Divide the second term by the binomial coefficient 3 Is the coefficient the cube root of the first term squared times the cube root

of the fourth term? 36x23=12x2=(4x2)(3)=(2x)2(3)

Divide the third term by the binomial coefficient 3 Is the coefficient the cube root of the first term times the cube root of the

fourth term squared? 54x3=18x=(2x)(9)=(2x)(3)2

The polynomial is the cube of a binomial 8x3+36x2+54x+27=(2x+3)3

Conclusions

The degree of the expansion is the exponent the binomial is raised to

The number of terms in the expansion is one more than the degree of the expansion

Pascal’s Triangle gives the binomial coefficients of the expansion

The binomial theorem is useful for large expansions

Questions