Exponential and Logarithmic Functions

Exponential and Logarithmic Functions

Chapter 4

Composite Functions

Section 4.1

Composite FunctionsConstruct new function from

two given functions f and gComposite function:

Denoted by f ° gRead as “f composed with g”Defined by

(f ° g)(x) = f(g(x))Domain: The set of all numbers

x in the domain of g such that g(x) is in the domain of f.

Composite Functions

Note that we perform the inside function g(x) first.

Composite Functions

Composite Functions

Example. Suppose that f(x) = x3 { 2 and g(x) = 2x2 + 1. Find the values of the following expressions.(a) Problem: (f ± g)(1)

Answer:(b) Problem: (g ± f)(1)

Answer: (c) Problem: (f ± f)(0)

Answer:

Composite Functions

Example. Suppose that f(x) = 2x2 + 3 and g(x) = 4x3 + 1.(a) Problem: Find f ± g.

Answer: (b) Problem: Find the domain of f ± g.

Answer: (c) Problem: Find g ± f.

Answer:(d) Problem: Find the domain of f ± g.

Answer:

Composite Functions Example. Suppose that f(x) =

and g(x) = (a) Problem: Find f ± g.

Answer:

(b) Problem: Find the domain of f ± g.

Answer:

(c) Problem: Find g ± f.

Answer:

(d) Problem: Find the domain of f ± g.

Answer:

Composite Functions

Example.

Problem: If f(x) = 4x + 2 and

g(x) = show that for all

x,

(f ± g)(x) = (g ± f)(x) = x

Decomposing Composite Functions

Example.

Problem: Find functions f and g

such that f ± g = H if

Answer:

Key Points

Composite FunctionsDecomposing Composite

Functions

One-to-One Functions;Inverse Functions

Section 4.2

One-to-One Functions

One-to-one function: Any two different inputs in the domain correspond to two different outputs in the range. If x1 and x2 are two different

inputs of a function f, then f(x1) f(x2).

One-to-One Functions

One-to-one function

Not a one-to-one function

Not a function

One-to-One Functions

Example.Problem: Is this function one-to-one?Answer:

Melissa

John

Jennifer

Patrick

$45,000

$40,000

$50,000

Person

Salary

One-to-One Functions

Example.Problem: Is this function one-to-one?Answer:

Alex

Kim

Dana

Pat

1451678

1672969

2004783

1914935

Person

ID Number

One-to-One Functions

Example. Determine whether the following functions are one-to-one.(a) Problem: f(x) = x2 + 2

Answer: (b) Problem: g(x) = x3 { 5

Answer:

One-to-One Functions

Theorem. A function that is increasing on an interval I is a one-to-one function on I.A function that is decreasing on an interval I is a one-to-one function on I.

Horizontal-line Test

If every horizontal line intersects the graph of a function f in at most one point, then f is one-to-one.

Horizontal-line Test

Example. Problem: Use the graph to

determine whether the function is one-to-one.

Answer:

-6 -4 -2 2 4 6

-6

-4

-2

2

4

6

Horizontal-line Test

Example. Problem: Use the graph to

determine whether the function is one-to-one.

Answer:

-6 -4 -2 2 4 6

-6

-4

-2

2

4

6

Inverse FunctionsRequires f to be a one-to-one

functionThe inverse function of f

Written f{1

Defined as the function which takes f(x) as inputReturns the output x.

In other words, f{1 undoes the action of f

f{1(f(x)) = x for all x in the domain of ff(f{1(x)) = x for all x in the domain of f{1

Inverse Functions

Example. Find the inverse of the function shown.Problem:

Alex

Kim

Dana

Pat

1451678

1672969

2004783

1914935

Person

ID Number

Inverse Functions

Example. (cont.)Answer:

Alex

Kim

Dana

Pat

1451678

1672969

2004783

1914935

Person

ID Number

Inverse Functions

Example. Problem: Find the inverse of the

function shown.f(0, 0), (1, 1), (2, 4), (3, 9), (4,

16)gAnswer:

Domain and Range of Inverse Functions

If f is one-to-one, its inverse is a function.

The domain of f{1 is the range of f.

The range of f{1 is the domain of f

Domain and Range of Inverse Functions

Example.

Problem: Verify that the inverse

of

f(x) = 3x { 1 is

Graphs of Inverse Functions

The graph of a function f and its inverse f{1 are symmetric with respect to the line y = x.

-6 -4 -2 2 4 6

-6

-4

-2

2

4

6

Graphs of Inverse Functions

Example. Problem: Find the graph of the

inverse functionAnswer:

Finding Inverse Functions

If y = f(x), Inverse if given implicitly by x =

f(y).Solve for y if possible to get y = f

{1(x) Process

Step 1: Interchange x and y to obtain an equation x = f(y)

Step 2: If possible, solve for y in terms of x.

Step 3: Check the result.

Finding Inverse Functions

Example. Problem: Find the inverse of the

function

Answer:

Restricting the Domain

If a function is not one-to-one, we can often restrict its domain so that the new function is one-to-one.

-6 -4 -2 2 4 6

-6

-4

-2

2

4

6

Restricting the Domain

Example. Problem: Find the inverse of

if the domain of f is x ¸ 0.Answer:

Key Points

One-to-One FunctionsHorizontal-line Test Inverse FunctionsDomain and Range of

Inverse FunctionsGraphs of Inverse FunctionsFinding Inverse FunctionsRestricting the Domain

Exponential Functions

Section 4.3

Exponents

For negative exponents:

For fractional exponents:

Exponents

Example. Problem: Approximate 3¼ to five

decimal places.Answer:

Laws of Exponents

Theorem. [Laws of Exponents]If s, t, a and b are real numbers with a > 0 and b > 0, then as ¢ at = as+t

(as)t = ast

(ab)s = as ¢ bs

1s = 1 a0 = 1

Exponential Functions

Exponential function: function of the form

f(x) = ax

where a is a positive real number (a > 0) a 1.

Domain of f: Set of all real numbers.

Warning! This is not the same as a power function. (A function of the form f(x) = xn)

Exponential Functions

Theorem. For an exponential function f(x) = ax, a > 0, a 1, if x is any real number, then

-6 -4 -2 2 4 6

-6

-4

-2

2

4

6

Graphing Exponential Functions

Example. Problem: Graph f(x) = 3x

Answer:

Graphing Exponential Functions

Properties of the Exponential Function

Properties of f(x) = ax, a > 1 Domain: All real numbers Range: Positive real numbers; (0, 1) Intercepts:

No x-intercepts y-intercept of y = 1

x-axis is horizontal asymptote as x {1

Increasing and one-to-one. Smooth and continuous Contains points (0,1), (1, a) and

Properties of the Exponential Function

f(x) = ax, a > 1

Properties of the Exponential Function

Properties of f(x) = ax, 0 < a < 1 Domain: All real numbers Range: Positive real numbers; (0, 1) Intercepts:

No x-intercepts y-intercept of y = 1

x-axis is horizontal asymptote as x 1

Decreasing and one-to-one. Smooth and continuous Contains points (0,1), (1, a) and

Properties of the Exponential Function

f(x) = ax, 0 < a < 1

The Number e

Number e: the number that the expression

approaches as n 1.Use ex or exp(x) on your

calculator.

The Number e

Estimating value of e n = 1: 2 n = 2: 2.25 n = 5: 2.488 32 n = 10: 2.593 742 460 1 n = 100: 2.704 813 829 42 n = 1000: 2.716 923 932 24 n = 1,000,000,000: 2.718 281 827

10 n = 1,000,000,000,000: 2.718 281

828 46

Exponential Equations

If au = av, then u = vAnother way of saying that

the function f(x) = ax is one-to-one.

Examples. (a) Problem: Solve 23x {1 = 32

Answer: (b) Problem: Solve

Answer:

Key Points

ExponentsLaws of ExponentsExponential FunctionsGraphing Exponential

FunctionsProperties of the Exponential

FunctionThe Number eExponential Equations

Logarithmic Functions

Section 4.4

Logarithmic Functions

Logarithmic function to the base aa > 0 and a 1Denoted by y = logaxRead “logarithm to the base a of

x” or “base a logarithm of x”Defined: y = logax if and only if x =

ay

Inverse function of y = ax

Domain: All positive numbers (0,1)

Logarithmic Functions

Examples. Evaluate the following logarithms

(a) Problem: log7 49

Answer:

(b) Problem:

Answer:

(c) Problem:

Answer:

Logarithmic Functions

Examples. Change each exponential expression to an equivalent expression involving a logarithm

(a) Problem: 2¼ = s

Answer:

(b) Problem: ed = 13

Answer:

(c) Problem: a5 = 33

Answer:

Logarithmic Functions

Examples. Change each logarithmic expression to an equivalent expression involving an exponent.

(a) Problem: loga 10 = 7

Answer:

(b) Problem: loge t = 4

Answer:

(c) Problem: log5 17 = z

Answer:

Domain and Range of Logarithmic Functions Logarithmic function is inverse of

the exponential function. Domain of the logarithmic

functionSame as range of the exponential

functionAll positive real numbers, (0, 1)

Range of the logarithmic functionSame as domain of the exponential

functionAll real numbers, ({1, 1)

Domain and Range of Logarithmic Functions

Examples. Find the domain

of each function

(a) Problem: f(x) = log9(4 { x2)

Answer:

(b) Problem:

Answer:

-6 -4 -2 2 4 6

-6

-4

-2

2

4

6

Graphing Logarithmic Functions

Example. Graph the function

Problem: f(x) = log3 x

Answer:

Properties of the Logarithmic Function

Properties of f(x) = loga x, a > 1Domain: Positive real numbers;

(0, 1)Range: All real numbersIntercepts:

x-intercept of x = 1No y-intercepts

y-axis is horizontal asymptoteIncreasing and one-to-one.Smooth and continuousContains points (1,0), (a, 1) and

Properties of the Logarithmic Function

Properties of the Logarithmic Function

Properties of f(x) = loga x, 0 < a < 1Domain: Positive real numbers; (0,

1)Range: All real numbers Intercepts:

x-intercept of x = 1No y-intercepts

y-axis is horizontal asymptoteDecreasing and one-to-one.Smooth and continuousContains points (1,0), (a, 1) and

Properties of the Logarithmic Function

Special Logarithm Functions

Natural logarithm:y = ln x if and only if x = ey

ln x = loge x

Common logarithm:y = log x if and only if x = 10y

log x = log10 x

-6 -4 -2 2 4 6

-6

-4

-2

2

4

6

Special Logarithm Functions

Example. Graph the function

Problem: f(x) = ln (3{x)

Answer:

Logarithmic Equations

Examples. Solve the logarithmic equations. Give exact answers.(a) Problem: log4 x = 3

Answer:

(b) Problem: log6(x{4) = 3

Answer:

(c) Problem: 2 + 4 ln x = 10

Answer:

Logarithmic Equations

Examples. Solve the exponential equations using logarithms. Give exact answers.(a) Problem: 31+2x= 243

Answer:

(b) Problem: ex+8 = 3

Answer:

Key Points

Logarithmic FunctionsDomain and Range of

Logarithmic FunctionsGraphing Logarithmic

FunctionsProperties of the Logarithmic

FunctionSpecial Logarithm FunctionsLogarithmic Equations

Properties of Logarithms

Section 4.5

Properties of Logarithms

Theorem. [Properties of Logarithms] For a > 0, a 1, and r some real number: loga 1 = 0

loga a = 1

loga ar = r

Properties of Logarithms

Theorem. [Properties of Logarithms] For M, N, a > 0, a 1, and r some real number:

loga (MN) = loga M + loga N

loga Mr = r loga M

Properties of Logarithms

Examples. Evaluate the following expressions.(a) Problem:

Answer: (b) Problem: log140 10 + log140 14

Answer:

(c) Problem: 2 ln e2.42

Answer:

Properties of Logarithms

Examples. Evaluate the following expressions if logb A = 5 and logbB = {4.

(a) Problem: logb AB

Answer: (b) Problem:

Answer: (c) Problem:

Answer:

Properties of Logarithms

Example. Write the following expression as a sum of logarithms. Express all powers as factors.

Problem:

Answer:

Properties of Logarithms

Example. Write the following expression as a single logarithm.

Problem: loga q { loga r + 6 loga

p

Answer:

Properties of Logarithms

Theorem. [Properties of Logarithms] For M, N, a > 0, a 1,If M = N, then loga M = loga N

If loga M = loga N, then M = N

Comes from fact that exponential and logarithmic functions are inverses.

Logarithms with Bases Other than e

and 10Example.

Problem: Approximate log3 19 rounded to four decimal places

Answer:

Logarithms with Bases Other than e

and 10Theorem. [Change-of-Base

Formula]If a 1, b 1 and M are all positive real numbers, then

In particular,

Logarithms with Bases Other than e and 10

Examples. Approximate the following logarithms to four decimal places

(a) Problem: log6.32 65.16

Answer:

(b) Problem:

Answer:

Key Points

Properties of LogarithmsProperties of LogarithmsLogarithms with Bases Other

than e and 10

Logarithmic and Exponential Equations

Section 4.6

Solving Logarithmic Equations

Example. Problem: Solve log3 4 = 2 log3 x

algebraically.Answer:

Solving Logarithmic Equations

Example. Problem: Solve log3 4 = 2 log3 x

graphically.Answer:

Solving Logarithmic Equations

Example. Problem: Solve log2(x+2) +

log2(1{x) = 1 algebraically.Answer:

Solving Logarithmic Equations

Example. Problem: Solve log2(x+2) +

log2(1{x) = 1 graphically.

Answer:

Solving Exponential Equations

Example. Problem: Solve 9x { 3x { 6 = 0

algebraically.Answer:

Solving Exponential Equations

Example. Problem: Solve 9x { 3x { 6 = 0

graphically.Answer:

Solving Exponential Equations

Example. Problem: Solve 3x = 7

algebraically. Give an exact answer, then approximate your answer to four decimal places.

Answer:

Solving Exponential Equations

Example. Problem: Solve 3x = 7

graphically. Approximate your answer to four decimal places.

Answer:

Solving Exponential Equations

Example. Problem: Solve 5 ¢ 2x = 3

algebraically. Give an exact answer, then approximate your answer to four decimal places.

Answer:

Solving Exponential Equations

Example. Problem: Solve 5 ¢ 2x = 3

graphically. Approximate your answer to four decimal places.

Answer:

Solving Exponential Equations

Example. Problem: Solve 2x{1 = 52x+3

algebraically. Give an exact answer, then approximate your answer to four decimal places.

Answer:

Solving Exponential Equations

Example. Problem: Solve e2x { x2 = 3

graphically. Approximate your answer to four decimal places.

Answer:

Key Points

Solving Logarithmic Equations

Solving Exponential Equations

Compound Interest

Section 4.7

Simple Interest

Simple Interest FormulaPrincipal of P dollars borrowed

for t years at per annum interest rate r

Interest is I = Prtr must be expressed as

decimal

Compound Interest

Payment periodAnnually: Once per yearSemiannually: Twice per yearQuarterly: Four times per yearMonthly: 12 times per yearDaily: 365 times per year

Compound Interest

Theorem. [Compound Interest Formula]The amount A after t years due to a principal P invested at an annual interest rate r compounded n times per year is

Compound Interest

Example. Find the amount that results from the investment of $1000 at 8% after a period of 8 years.(a) Problem: Compounded annually

Answer:(b) Problem: Compounded quarterly

Answer:(c) Problem: Compounded daily

Answer:

Compound Interest

Theorem. [Continuous Compounding]The amount A after t years due to a principal P invested at an annual interest rate r compounded continuously is

Compound Interest

Example. Find the amount that results from the investment of $1000 at 8% after a period of 8 years.Problem: Compounded

continuouslyAnswer:

Effective Rates of Interest

Effective Rate of Interest: Equivalent annual simple interest rate that yields same amount as compounding after 1 year.

Effective Rates of Interest

Example. Find the effective rate of interest on an investment at 8%(a) Problem: Compounded monthly

Answer:(a) Problem: Compounded daily

Answer:(a) Problem: Compounded

continuously Answer:

Present Value

Present value: amount needed to invest now to receive A dollars at a specified future time.

Present Value

Theorem. [Present Value Formulas]The present value P of A dollars to be received after t years, assuming a per annum interest rate r compounded n times per year, is

if the interest is compounded continuously, then

Present Value

Example.Problem: Find the present value

of $5600 after 4 years at 10% compounded semiannually. Round to the nearest cent.

Answer:

Time to Double an Investment

Example.Problem: What annual rate of

interest is required to double an investment in 8 years?

Answer:

Key Points

Simple InterestCompound InterestEffective Rates of InterestPresent ValueTime to Double an

Investment

Exponential Growth and Decay; Newton’s Law; Logistic Growth and Decay

Section 4.8

Uninhibited Growth and Decay

Uninhibited Growth:No restriction to growthExamples

Cell division (early in process)Compound Interest

Uninhibited DecayExamples

Radioactive decay Compute half-life

Uninhibited Growth and Decay

Uninhibited Growth:N(t) = N0 ekt, k > 0

N0: initial population k: positive constant t: time

Uninhibited DecayA(t) = A0 ekt, k < 0

N0: initial amount k: negative constant t: time

Uninhibited Growth and Decay

Example.Problem: The size P of a small

herbivore population at time t (in years) obeys the function P(t) = 600e0.24t if they have enough food and the predator population stays constant. After how many years will the population reach 1800?

Answer:

Uninhibited Growth and Decay

Example.Problem: The half-life of carbon

14 is 5600 years. A fossilized leaf contains 12% of its normal amount of carbon 14. How old is the fossil (to the nearest year)?

Answer:

Newton’s Law of Cooling

Temperature of a heated object decreases exponentially toward temperature of surrounding medium

Newton’s Law of CoolingThe temperature u of a heated object at a given time t can be modeled by

u(t) = T + (u0 { T)ekt, k < 0where T is the constant temperature of the surrounding medium, u0 is the initial temperature of the heated object, and k is a negative constant.

Newton’s Law of Cooling

Example.Problem: The temperature of a dead

body that has been cooling in a room set at 70±F is measured as 88±F. One hour later, the body temperature is 87.5±F. How long (to the nearest hour) before the first measurement was the time of death, assuming that body temperature at the time of death was 98.6±F?

Answer:

Logistic Model

Uninhibited growth is limited in actuality

Growth starts off like exponential, then levels off

This is logistic growthPopulation approaches

carrying capacity

Logistic ModelLogistic Model

In a logistic growth model, the population P after time t obeys the equation

where a, b and c are constants with c > 0 (c is the carrying capacity). The model is a growth model if b > 0; the model is a decay model if b < 0.

Logistic Model

Logistic Model

Properties of Logistic FunctionDomain is set of all real numbersRange is interval (0, c)Intercepts:

no x-intercepty-intercept is P(0).

Increasing if b > 0, decreasing if b < 0

Inflection point when P(t) = 0.5cGraph is smooth and continuous

Logistic Model Example. The logistic growth model

represents the population of a species introduced into a new territory after t years.(a) Problem: What was the initial

population introduced? Answer:

(b) Problem: When will the population reach 80?

Answer: (c) Problem: What is the carrying

capacity? Answer:

Key Points

Uninhibited Growth and Decay

Newton’s Law of CoolingLogistic Model

Building Exponential, Logarithmic, and Logistic Models from Data Section 4.9

Fitting an Exponential Function to Data

Example. The population (in hundred thousands) for the Colonial US in ten-year increments for the years 1700-1780 is given in the following table. (Source: 1998 Information Please Almanac)

Decade, x

Population, P

0 251

1 332

2 466

3 629

4 906

5 1171

6 1594

7 2148

8 2780

Fitting an Exponential Function to Data

Example. (cont.)(a) Problem: State whether the

data can be more accurately modeled using an exponential or logarithmic function.

Answer:

Fitting an Exponential Function to Data

Example. (cont.)(b) Problem: Find a model for

population (in hundred thousands) as a function of decades since 1700.

Answer:

Fitting a Logarithmic Function to Data

Example. The death rate (in deaths per 100,000 population) for 20-24 year olds in the US between 1985-1993 are given in the following table. (Source: NCHS Data Warehouse)

Year Rate of Death, r

1985 134.9

1987 154.7

1989 162.9

1991 174.5

1992 182.2

Fitting a Logarithmic Function to Data

Example. (cont.)(a) Problem: Find a model for

death rate in terms of x, where x denotes the number of years since 1980.

Answer: (b) Problem: Predict the year in

which the death rate first exceeded 200.

Answer:

Fitting a Logistic Function to Data

Example. A mechanic is testing the cooling system of a boat engine. He measures the engine’s temperature over time.

Time t (min.)

Temperature T (±F)

5 100

10 180

15 270

20 300

25 305

Fitting a Logistic Function to Data

Example. (cont.)(a) Problem: Find a model for

the temperature T in terms of t, time in minutes.

Answer:

(b) Problem: What does the model imply will happen to the temperature as time passes?

Answer:

Key Points

Fitting an Exponential Function to Data

Fitting a Logarithmic Function to Data

Fitting a Logistic Function to Data