Functions and Logarithms
One-to-One FunctionsA function f(x) is one-to-one if
f(a) ≠ f(b) whenever a ≠ b.Must pass horizontal line test.
Not one-to-oneOne-to-one
InversesIf a function is one-to-one, then it
has an inverse.◦Notation: f-1(x)◦IMPORTANT:
!!!!!)(
1meannot does )(1
xfxf
InversesTo find the inverse of a function,
solve for x, “swap” x and y, and solve for y.
Ex: What is the inverse of f(x) = -2x + 4?
y = -2x + 4y – 4 = -2x
xy
2
4
yx
2
4(“swap” x and y)
yx 22
1
Therefore,
22
1)(1 xxf
InversesInverse functions are symmetric
(reflected) about the line y = x.Therefore, in order for two
functions to be inverses, the results of the composites is x.
x x f f x x f f
)) ( ( AND )) ( (1 1
InversesExample: Prove the two
functions from our last example are inverses. 2
2
1)( and 42)( 1
xxfxxf
22
1))(( 1 xfxff
422
12
x
422
12
x
44 x
= x
Now, we must check the other composite!
42))(( 11 xfxff
2)42(2
1
x
22 x
= xTherefore, these two functions must be inverses of one another!!!
Logarithmsy = logax means ay = x
◦Ex: log381 = 4 means 34 = 81
What is the inverse of y = logax?Since y = logax is ay = x, then the inverse has to beax = y
LogarithmsCommon logarithms:
log x means log10x
ln x means logex
Inverse Properties of Logs
xa xa log xa xa log
(both of these hold true if a > 1 and x > 0)
xe x ln xex ln(both of these hold true if x > 0)
Inverse Properties of LogsExample:
Solve ln x = 3t + 5 for x.
53ln tx53ln tx ee (use each side as an exponent
of e)53 tex (e and ln are inverses and “undo”
each other.)
Inverse Property of LogsExample:
Solve e2x = 10 for x.
102 xe10lnln 2 xe (take the natural log of both
sides)
10ln2 x (ln and e are inverses and “undo” each other.)
151.12
10lnx
Properties of LogarithmsFor any real numbers x > 0 and y
> 0,xyyx aaa logloglog
y
xyx aaa logloglog
xyx ay
a loglog
Change of Base PropertySince our calculators will not
calculate logs of bases other than 10 or e,
a
xxa ln
lnlog
ExampleSarah invests $1000 in an account that
earns 5.25% interest compounded annually. How long will it take the account to reach $2500?t)0525.01(10002500
t)0525.1(5.2 (divide both sides by 1000)
t)0525.1ln()5.2ln( (take a log of both sides…doesn’t matter what base you use!!!)
)0525.1ln()5.2ln( t (by my property, exponent comes out front)
)0525.1ln(
)5.2ln(t (divide by ln(1.0525))t ≈ 17.907
years
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