# Modeling, Functions, and Graphs

## Section1.2Functions

### SubsectionDefinition of Function

We often want to predict values of one variable from the values of a related variable. For example, when a physician prescribes a drug in a certain dosage, she needs to know how long the dose will remain in the bloodstream. A sales manager needs to know how the price of his product will affect its sales. A function is a special type of relationship between variables that allows us to make such predictions.
Suppose it costs $800 for flying lessons, plus$30 per hour to rent a plane. If we let $$C$$ represent the total cost for $$t$$ hours of flying lessons, then
\begin{equation*} C=800+30t ~~~~ (t\ge 0) \end{equation*}
Thus, for example
 when $$t=\alert{0}\text{,}$$ $$C=800+30(\alert{0})=800$$ when $$t=\alert{4}\text{,}$$ $$C=800+30(\alert{4})=920$$ when $$t=\alert{10}\text{,}$$ $$C=800+30(\alert{10})=1100$$
The variable $$t$$ is called the input or independent variable, and $$C$$ is the output or dependent variable, because its values are determined by the value of $$t\text{.}$$ We can display the relationship between two variables by a table or by ordered pairs. The input variable is the first component of the ordered pair, and the output variable is the second component.
 $$t$$ $$C$$ $$(t,C)$$ $$0$$ $$800$$ $$(0, 800)$$ $$4$$ $$920$$ $$(4, 920)$$ $$10$$ $$1100$$ $$(10,1100)$$
For this relationship, we can find the value of $$C$$ for any given value of $$t\text{.}$$ All we have to do is substitute the value of $$t$$ into the equation and solve for $$C\text{.}$$ Note that there can be only one value of $$C$$ for each value of $$t\text{.}$$

#### Definition of Function.

A function is a relationship between two variables for which a unique value of the output variable can be determined from a value of the input variable.

#### Note1.33.

What distinguishes functions from other variable relationships? The definition of a function calls for a unique value—that is, exactly one value of the output variable corresponding to each value of the input variable. This property makes functions useful in applications because they can often be used to make predictions.

#### Example1.34.

1. The distance, $$d\text{,}$$ traveled by a car in 2 hours is a function of its speed, $$r\text{.}$$ If we know the speed of the car, we can determine the distance it travels by the formula $$d = r \cdot 2\text{.}$$
2. The cost of a fill-up with unleaded gasoline is a function of the number of gallons purchased. The gas pump represents the function by displaying the corresponding values of the input variable (number of gallons) and the output variable (cost).
3. Score on the Scholastic Aptitude Test (SAT) is not a function of score on an IQ test, because two people with the same score on an IQ test may score differently on the SAT; that is, a person’s score on the SAT is not uniquely determined by his or her score on an IQ test.

#### Checkpoint1.35.Practice 1.

1. As part of a project to improve the success rate of freshmen, the counseling department studied the grades earned by a group of students in English and algebra. Do you think that a student’s grade in algebra is a function of his or her grade in English?
• Yes
• No
Explain why or why not.
• A) Each value of $$x$$ has exactly one value of $$y$$ associated with it.
• B) Two students with the same grade English can have different grades in algebra.
• C) Two students with the same grade math will also have the same grade in English.
• D) Two students with the same grade math can have different grades in English.
2. Phatburger features a soda bar, where you can serve your own soft drinks in any size. Do you think that the number of calories in a serving of Zap Kola is a function of the number of fluid ounces?
• Yes
• No
Explain why or why not.
• A) The number of calories is proportional to the number of fluid ounces.
• B) Two servings with the same calories will have different fluid ounces.
• C) Two servings with the same flid ounces will have different calories.
$$\text{No}$$
$$\text{Choice 2}$$
$$\text{Yes}$$
$$\text{A) The ... fluid ounces.}$$
Solution.
1. No, students with the same grade in English can have different grades in algebra.
2. Yes, the number of calories is proportional to the number of fluid ounces.

#### Checkpoint1.36.QuickCheck 1.

What distinguishes a function from other variable relationships?
• A) The variables are related by a formula.
• B) The values of the input and output variables must be different.
• C) There cannot be two output values for a single input value.
• D) There cannot be two input values for a single output value.
$$\text{C) There ... input value.}$$
Solution.
There cannot be two output values for a single input value.
A function can be described in several different ways. In the following examples, we consider functions defined by tables, by graphs, and by equations.

### SubsectionFunctions Defined by Tables

When we use a table to describe a function, the first variable in the table (the left column of a vertical table or the top row of a horizontal table) is the input variable, and the second variable is the output. We say that the output variable is a function of the input.

#### Example1.37.

1. The table below shows data on sales compiled over several years by the accounting office for Eau Claire Auto Parts, a division of Major Motors. In this example, the year is the input variable, and total sales is the output. We say that total sales, $$S\text{,}$$ is a function of $$t\text{.}$$
 Year $$(t)$$ Total sales $$(S)$$ 2000 $612,000 2001$663,000 2002 $692,000 2003$749,000 2004 $904,000 2. The table below gives the cost of sending printed material by first-class mail starting July 10, 2022.  Weight in ounces $$(w)$$ Postage $$(P)$$ $$0 \lt w \le 1$$$0.60 $$1 \lt w \le 2$$ $0.84 $$2 \lt w \le 3$$$1.08 $$3 \lt w \le 4$$ $1.32 $$4 \lt w \le 5$$$1.56 $$5 \lt w \le 6$$ $1.80 $$6 \lt w \le 7$$$2.04

#### Checkpoint1.39.Practice 2.

Decide whether each table describes $$y$$ as a function of $$x\text{.}$$ Explain your choice.
1.  $$x$$ $$3.5$$ $$2.0$$ $$2.5$$ $$3.5$$ $$2.5$$ $$4.0$$ $$2.5$$ $$3.0$$ $$y$$ $$2.5$$ $$3.0$$ $$2.5$$ $$4.0$$ $$3.5$$ $$4.0$$ $$2.0$$ $$2.5$$
Is $$y$$ a function of $$x\text{?}$$
• Yes
• No
• Each value of $$x$$ has exactly one value of $$y$$ associated with it.
• For example, $$x=3.5$$ corresponds both to $$y=2.5$$ and also to $$y=4.0$$
1.  $$x$$ $$-3$$ $$-2$$ $$-1$$ $$0$$ $$1$$ $$2$$ $$3$$ $$y$$ $$17$$ $$3$$ $$0$$ $$-1$$ $$0$$ $$3$$ $$17$$
Is $$y$$ a function of $$x\text{?}$$
• Yes
• No
• Each value of $$x$$ has exactly one value of $$y$$ associated with it.
• For example, $$y=3$$ corresponds both to $$x=-2$$ and also to $$x=2$$
$$\text{No}$$
$$\text{Choice 2}$$
$$\text{Yes}$$
$$\text{Choice 1}$$
Solution.
1. No, for example, $$x=3.5$$ corresponds both to $$y=2.5$$ and also to $$y=4\text{.}$$
2. Yes, each value of $$x$$ has exactly one value of $$y$$ associated with it.

#### Checkpoint1.40.QuickCheck 2.

How would you know if a table of values does not come from a function?
• A) The output values are all the same.
• B) The input values are not evely spaced.
• C) Two different input values have the same output value.
• D) Two different output values have the same input value.
$$\text{D) Two ... input value.}$$
Solution.
Two different output values have the same input value.

### SubsectionFunctions Defined by Graphs

We can also use a graph to define a function. The input variable is displayed on the horizontal axis, and the output variable on the vertical axis.

#### Example1.41.

The graph shows the number of hours, $$H\text{,}$$ that the sun is above the horizon in Peoria, Illinois, on day $$t\text{,}$$ where $$t = 0$$ on January 1.
1. Which variable is the input, and which is the output?
2. How many hours of sunlight are there in Peoria on day 150?
3. On which days are there 12 hours of sunlight?
4. What are the maximum and minimum values of $$H\text{,}$$ and when do these values occur?
Solution.
1. The input variable, $$t\text{,}$$ appears on the horizontal axis. The number of daylight hours, $$H\text{,}$$ is a function of the date. The output variable appears on the vertical axis.
2. The point on the curve where $$t = 150$$ has $$H \approx 14.1\text{,}$$ so Peoria gets about 14.1 hours of daylight when $$t = 150\text{,}$$ which is at the end of May.
3. $$H = 12$$ at the two points where $$t \approx 85$$ (in late March) and $$t \approx 270$$ (late September).
4. The maximum value of 14.4 hours occurs on the longest day of the year, when $$t \approx 170\text{,}$$ about three weeks into June. The minimum of 9.6 hours occurs on the shortest day, when $$t \approx 355\text{,}$$ about three weeks into December.

#### Checkpoint1.42.Practice 3.

The graph shows the elevation in feet, $$a\text{,}$$ of the Los Angeles Marathon course at a distance $$d$$ miles into the race. (Source: Los Angeles Times, March 3, 2005)
1. Which variable is the input, and which is the output?
• The input variable is $$d \text{,}$$ and the output variable is $$a \text{.}$$
• The input variable is $$a \text{,}$$ and the output variable is $$d \text{.}$$
2. What is the elevation at mile 20?
3. At what distances is the elevation 150 feet?
The relevant distances (to the nearest half-mile) separated by commas: miles
4. What are the maximum and minimum values of $$a \text{,}$$ and when do these values occur?
The maximum elevation is $$a=$$ feet which occurs at $$d=$$.
5. The runners pass by the Los Angeles Coliseum at about 4.2 miles into the race. What is the elevation there?
Approximately (within 5) feet
$$\text{Choice 1}$$
$$210$$
$$5, 11, 12, 16, 17.5, 18$$
$$300$$
$$0$$
$$165$$
Solution.
1. The input variable is $$d \text{,}$$ and the output variable is $$a \text{.}$$
2. Approximately 210 feet
3. Approximately where $$d\approx 5 \text{,}$$ $$d\approx 11 \text{,}$$ $$d\approx 12 \text{,}$$ $$d\approx 16 \text{,}$$ $$d\approx 17.5 \text{,}$$ and $$d\approx 18$$
4. The maximum value of 300 feet occurs at the start, when $$d = 0 \text{.}$$ The minimum of 85 feet occurs when $$d\approx 15 \text{.}$$
5. Approximately 165 feet

### SubsectionFunctions Defined by Equations

Example 1.43 illustrates a function defined by an equation.

#### Example1.43.

As of 2020, One World Trade Center in New York City is the nation’s tallest building, at 1776 feet. If an algebra book is dropped from the top of One World Trade Center, its height above the ground after $$t$$ seconds is given by the equation
\begin{equation*} h = 1776 - 16t^2 \end{equation*}
Thus, after $$\alert{1}$$ second the book’s height is
\begin{equation*} h = 1776 - 16(\alert{1})^2 = 1760 \text{ feet} \end{equation*}
After $$\alert{2}$$ seconds its height is
\begin{equation*} h = 1776 - 16(\alert{2})^2 = 1712 \text{ feet} \end{equation*}
For this function, $$t$$ is the input variable and $$h$$ is the output variable. For any value of $$t\text{,}$$ a unique value of $$h$$ can be determined from the equation for $$h\text{.}$$ We say that $$h$$ is a function of $$t\text{.}$$

#### Checkpoint1.44.Practice 4.

Write an equation that gives the volume, $$V\text{,}$$ of a sphere as a function of its radius, $$r\text{.}$$
$$V=$$
$${\frac{4}{3}}\pi r^{3}$$
Solution.
$$V=\dfrac{4}{3}\pi r^3$$

#### Checkpoint1.45.QuickCheck 3.

Name three ways to describe a function.
• A) By inputs, outputs, or evaluation
• B) By tables, equations, or graphs
• C) By the intercepts, the slope, or the vertex
• D) By numbers, letters, or diagrams
$$\text{B) By ... , or graphs}$$
Solution.
By tables, equations, or graphs

#### Technology1.46.Making a Table of Values with a Calculator.

We can use a graphing calculator to make a table of values for a function defined by an equation. For the function in Example 1.43,
\begin{equation*} h = 1776 - 16t^2 \end{equation*}
• Enter the equation: Press the Y= key, clear out any other equations, and define $$Y_1 = 1776 - 16X^2.$$
• Choose the $$x$$-values for the table. Press 2ndWINDOW to access the $$TblSet$$ (Table Setup) menu and set it to look like the figure at left below.
This setting will give us an initial x-value of 0 $$(TblStart = 0)$$ and an increment of one unit in the $$x$$-values, $$(\Delta Tbl = 1)\text{.}$$ It also fills in values of both variables automatically.
• Press 2nd GRAPH to see the table of values, as shown in the figure at right below. From this table, we can check the heights we found in Example 1.43.
Now try making a table of values with $$TblStart = 0$$ and $$\Delta Tbl = 0.5\text{.}$$ Use the and arrow keys to scroll up and down the table.

#### Checkpoint1.47.Pause and Reflect.

Write one question you still have about functions.

### SubsectionFunction Notation

There is a convenient notation for discussing functions. First, we choose a letter, such as $$f\text{,}$$ $$g\text{,}$$ or $$h$$ (or $$F\text{,}$$ $$G\text{,}$$ or $$H$$), to name a particular function. (We can use any letter, but these are the most common choices.)
For instance, in Example 1.43, the height, $$h\text{,}$$ of a falling algebra book is a function of the elapsed time, $$t\text{.}$$ We might call this function $$f\text{.}$$ In other words, $$f$$ is the name of the relationship between the variables $$h$$ and $$t\text{.}$$ We write
\begin{equation*} h = f (t) \end{equation*}
which means "$$h$$ is a function of $$t\text{,}$$ and $$f$$ is the name of the function."

#### Caution1.48.

The new symbol $$f(t)\text{,}$$ read "$$f$$ of $$t\text{,}$$" is another name for the height, $$h\text{.}$$ The parentheses in the symbol $$f(t)$$ do not indicate multiplication. (It would not make sense to multiply the name of a function by a variable.) Think of the symbol $$f(t)$$ as a single variable that represents the output value of the function.
With this new notation we may write
\begin{equation*} h = f (t) = 1776 - 16t^2 \end{equation*}
or just
\begin{equation*} f (t) = 1776 - 16t^2 \end{equation*}
\begin{equation*} h = 1776 - 16t^2 \end{equation*}
to describe the function.

#### Note1.49.

Perhaps it seems complicated to introduce a new symbol for $$h\text{,}$$ but the notation $$f(t)$$ is very useful for showing the correspondence between specific values of the variables $$h$$ and $$t\text{.}$$

#### Example1.50.

In Example 1.43, the height of an algebra book dropped from the top of One World Trade Center is given by the equation
\begin{equation*} h = 1776 - 16t^2 \end{equation*}
We see that
 when $$t=1$$ $$h=1760$$ when $$t=2$$ $$h=1712$$
Using function notation, these relationships can be expressed more concisely as
 $$f(1)=1760$$ and $$f(2)=1712$$
which we read as "$$f$$ of 1 equals 1760" and "$$f$$ of 2 equals 1712." The values for the input variable, $$t\text{,}$$ appear inside the parentheses, and the values for the output variable, $$h\text{,}$$ appear on the other side of the equation.
Remember that when we write $$y = f(x)\text{,}$$ the symbol $$f(x)$$ is just another name for the output variable.

#### Checkpoint1.51.QuickCheck 4.

True or False.
1. The notation $$f(t)$$ indicates the product of $$f$$ and $$t\text{.}$$
• True
• False
2. If $$y=f(x)\text{,}$$ then $$f(x)$$ gives the value of the input variable.
• True
• False
3. If $$Q$$ is a function of $$M\text{,}$$ we may write $$M=f(Q)\text{.}$$
• True
• False
4. In the equation $$d=g(n)$$ the letters $$d,~g,$$ and $$n$$ are variables.
• True
• False
$$\text{False}$$
$$\text{False}$$
$$\text{False}$$
$$\text{False}$$
Solution.
1. False
2. False
3. False
4. False

#### Checkpoint1.52.Practice 5.

Let $$F$$ be the name of the function defined by the graph in Example 1.41, the number of hours of daylight in Peoria $$t$$ days after January 1.
1. Use function notation to state that $$H$$ is a function of $$t\text{.}$$
• $$\displaystyle F=H(t)$$
• $$\displaystyle H=F(t)$$
• $$\displaystyle t=F(H)$$
• $$\displaystyle H=t(F)$$
2. What does the statement $$F(15) = 9.7$$ mean in the context of the problem?
• A) The sun is 9.7 degrees above the horizon in Peoria on January 15.
• B) The sun is above the horizon in Peoria for 15 hours on January 10.
• C) The sun is above the horizon in Peoria for 9.7 hours on January 16.
$$\text{Choice 2}$$
$$\text{C) The ... January 16.}$$
Solution.
1. $$\displaystyle H = F(t)$$
2. The sun is above the horizon in Peoria for 9.7 hours on January 16.

#### Checkpoint1.53.QuickCheck 4.

Use function notation to write the statement “$$L$$ defines $$w$$ as a function of $$p\text{.}$$
• $$\displaystyle L=w(p)$$
• $$\displaystyle w=L(p)$$
• $$\displaystyle p=L(w)$$
• $$\displaystyle L=p(w)$$
$$\text{Choice 2}$$
Solution.
$$w=L(p)$$

### SubsectionUsing Function Notation

Finding the value of the output variable that corresponds to a particular value of the input variable is called evaluating the function.

#### Example1.54.

Let $$g$$ be the name of the postage function defined by the table in Example 1.34 b. Find $$g(1)\text{,}$$ $$g(3)\text{,}$$ and $$g(6.75$$).
Solution.
According to the table,
 when $$w=1\text{,}$$ $$p=0.47$$ so $$g(1)=0.47$$ when $$w=3\text{,}$$ $$p=0.89$$ so $$g(3)=0.89$$ when $$w=6.75\text{,}$$ $$p=1.73$$ so $$g(6.75)=1.73$$

#### Checkpoint1.61.Practice 8.

The volume of a sphere of radius $$r$$ centimeters is given by
\begin{equation*} V = V(r) = \frac{4}{3}\pi r^3 \end{equation*}
Evaluate $$V(10)$$ and explain what it means.
$$V(10)=$$, which represents
• A) the volume (in cm) of a sphere whose radius is 10 cu. cm
• B) the radius (in cm) of a sphere whose volume is 10 cm
• C) the volume (in sq. cm) of a sphere whose radius is 10 cm
• D) the volume (in cu. cm) of a sphere whose radius is 10 cm
$$4188.79$$
$$\text{D) the ... is 10 cm}$$
Solution.
$$V(10) = 4000\pi/3\approx 4188.79 \text{ cm}^3$$ is the volume of a sphere whose radius is 10 cm.

#### Checkpoint1.62.Pause and Reflect.

Do linear equations $$y=mx+b$$ and quadratic equations $$y=ax^2+bx+c$$ define functions? Why or why not?

### SubsectionOperations with Function Notation

Sometimes we need to evaluate a function at an algebraic expression rather than at a specific number.

#### Example1.63.

TrailGear manufactures backpacks at a cost of
\begin{equation*} C(x) = 3000 + 20x \end{equation*}
for $$x$$ backpacks. The company finds that the monthly demand for backpacks increases by 50% during the summer. The backpacks are produced at several small co-ops in different states.
1. If each co-op usually produces $$b$$ backpacks per month, how many should it produce during the summer months?
2. What costs for producing backpacks should the company expect during the summer?
Solution.
1. An increase of 50% means an additional 50% of the current production level, $$b\text{.}$$ Therefore, a co-op that produced $$b$$ backpacks per month during the winter should increase production to $$b + 0.5b\text{,}$$ or $$1.5b$$ backpacks per month in the summer.
2. The cost of producing $$1.5b$$ backpacks will be

#### Checkpoint1.64.Practice 9.

A spherical balloon has a radius of 10 centimeters.
1. If we increase the radius by $$h$$ centimeters, what will the new volume be?
• $$V(10)+h=\frac{4}{3}\pi(10^3)+h$$ cu. cm
• $$V(10)+h=\frac{4}{3}\pi(10^{3+h})$$ cu. cm
• $$V(10+h)=\frac{4}{3}\pi\cdot 10^3+h$$ cu. cm
• $$V(10+h)=\frac{4}{3}\pi(10+h)^3$$ cu. cm
2. If $$h = 2 \text{,}$$ how much did the volume increase? Round your answer to hundredths.
It increased by $$\text{cm}^3 \text{.}$$
$$\text{Choice 4}$$
$$3049.44$$
Solution.
1. $$\displaystyle V(10 + h) = \dfrac{4}{3}\pi(10 + h)^3 \text{ cm}^3$$
2. From $$V(10)$$ to $$V(12)$$ is an increase of about $$3049.44 \text{ cm}^3$$

#### Example1.65.

Evaluate the function $$f(x)=4x^2 - x + 5$$ for the following expressions.
1. $$\displaystyle x = 2h$$
2. $$\displaystyle x = a + 3$$
Solution.
1.
\begin{aligned}[t] f(\alert{2h}) \amp= 4(\alert{2h})^2-(\alert{2h}) + 5\\ \amp= 4(4h^2)-2h+5\\ \amp= 16h^2 - 2h + 5\\ \end{aligned}
2.
\begin{aligned}[t] f(\alert{a+3}) \amp= 4(\alert{a+3})^2-(\alert{a+3})+5\\ \amp= 4(a^2+6a+9)-a-3+5\\ \amp= 4a^2+24a+36 - a + 2\\ \amp= 4a^2+23a + 38\\ \end{aligned}

#### Caution1.66.

In Example 1.65, notice that
\begin{equation*} f(2h) \ne 2 f(h) \end{equation*}
and
\begin{equation*} f(a + 3) \ne f(a) + f(3) \end{equation*}
To compute $$f(a) + f(3)\text{,}$$ we must first compute $$f(a)$$ and $$f(3)\text{,}$$ then add them:
\begin{equation*} \begin{aligned}[t] f(a)+f(3)\amp= (4a^2-a+5)+(4\cdot 3^2-3+5) \\ \amp= 4a^2 - a + 43\\ \end{aligned} \end{equation*}
In general, it is not true that $$f(a + b) = f(a) + f(b)\text{.}$$ Remember that the parentheses in the expression $$f(x)$$ do not indicate multiplication, so the distributive law does not apply to the expression $$f(a + b)\text{.}$$

#### Checkpoint1.67.QuickCheck 6.

Define the function $$f(x)=x^2\text{.}$$ Which of these is equal to $$f(x+y)\text{?}$$
• $$\displaystyle f(x)+f(y)$$
• $$\displaystyle f(x+y)^2$$
• $$\displaystyle x^2+y^2$$
• $$\displaystyle (x+y)^2$$
$$\text{Choice 4}$$
Solution.
$$(x+y)^2$$

#### Checkpoint1.68.Practice 10.

Let $$f(x) = x^3 - 1$$ and evaluate each expression.
1. $$f(2) + f(3)=$$
2. $$f(2 + 3)=$$
3. $$2 f(x) + 3$$
$$33$$
$$124$$
$$2\!\left(x^{3}-1\right)+3$$
Solution.
1. $$\displaystyle 33$$
2. $$\displaystyle 124$$
3. $$\displaystyle 2x^3 + 1$$

#### Checkpoint1.69.Pause and Reflect.

Explain why $$f(a+b)$$ is not the same as $$f(a)+f(b)$$ for most functions.

### SubsectionSection Summary

#### SubsubsectionVocabulary

Look up the definitions of new terms in the Glossary.
• Function
• Input variable
• Independent variable
• Function value
• Dependent variable
• Output variable

#### SubsubsectionCONCEPTS

1. A function is a rule that assigns to each value of the input variable a unique value of the output variable.
2. Functions may be defined by words, tables, graphs, or equations.
3. Function notation: $$y = f (x)\text{,}$$ where $$x$$ is the input and $$y$$ is the output.

#### SubsubsectionSTUDY QUESTIONS

1. What property makes a relation between two variables a function?
2. Name three ways to define a function.
3. Give an example of a function in which two distinct values of the input variable correspond to the same value of the output variable.
4. Use function notation to write the statement "$$G$$ defines $$w$$ as a function of $$p\text{.}$$"
5. Give an example of a function for which $$f (2 + 3)\ne f (2) + f (3)\text{.}$$

#### SubsubsectionSKILLS

Practice each skill in the Homework problems listed.
1. Decide whether a relationship between two variables is a function: #1–26
2. Evaluate a function defined by a table, a graph, or an equation: #27–54
3. Choose appropriate scales for the axes: #5–12
4. Interpret function notation: #31–34, 49–54
5. Simplify expressions involving function notation: #59–76

### ExercisesHomework 1.2

#### Exercise Group.

For which of Problems 1-6 is the second quantity a function of the first? Explain your answers.
##### 1.
Price of an item; sales tax on the item at 4%
##### 2.
Time traveled at constant speed; distance traveled
##### 3.
Number of years of education; annual income
##### 4.
Distance flown in an airplane; price of the ticket
##### 5.
Volume of a container of water; the weight of the water
##### 6.
Amount of a paycheck; amount of Social Security tax withheld

#### Exercise Group.

Each of the objects in Problems 7-14 establishes a correspondence between two variables. Suggest appropriate input and output variables and decide whether the relationship is a function.
##### 7.
An itemized grocery receipt
##### 8.
An inventory list
An index
A will
A bathroom scale

#### Exercise Group.

Which of the tables in Problems 15-26 define the second variable as a function of the first variable? Explain why or why not.
##### 15.
 $$x$$ $$t$$ $$-1$$ $$2$$ $$0$$ $$9$$ $$1$$ $$-2$$ $$0$$ $$-3$$ $$-1$$ $$5$$
##### 16.
 $$y$$ $$w$$ $$0$$ $$8$$ $$1$$ $$12$$ $$3$$ $$7$$ $$5$$ $$-3$$ $$7$$ $$4$$
##### 17.
 $$x$$ $$y$$ $$-3$$ $$8$$ $$-2$$ $$3$$ $$-1$$ $$0$$ $$0$$ $$-1$$ $$1$$ $$0$$ $$2$$ $$3$$ $$3$$ $$8$$
##### 18.
 $$s$$ $$t$$ $$2$$ $$5$$ $$4$$ $$10$$ $$6$$ $$15$$ $$8$$ $$20$$ $$6$$ $$25$$ $$4$$ $$30$$ $$2$$ $$35$$

#### Exercise Group.

##### 19.
 $$r$$ $$-4$$ $$-2$$ $$0$$ $$2$$ $$4$$ $$v$$ $$6$$ $$6$$ $$3$$ $$6$$ $$8$$
##### 20.
 $$p$$ $$-5$$ $$-4$$ $$-3$$ $$-2$$ $$-1$$ $$d$$ $$-5$$ $$-4$$ $$-3$$ $$-2$$ $$-1$$
##### 21.
 Pressure ($$p$$) Volume ($$v$$) $$15$$ $$100.0$$ $$20$$ $$75.0$$ $$25$$ $$60.0$$ $$30$$ $$50.0$$ $$35$$ $$42.8$$ $$40$$ $$37.5$$ $$45$$ $$33.3$$ $$50$$ $$30.0$$
##### 22.
 Frequency ($$f$$) Wavelength ($$w$$) $$5$$ $$60.0$$ $$10$$ $$30.0$$ $$20$$ $$15.0$$ $$30$$ $$10.0$$ $$40$$ $$7.5$$ $$50$$ $$6.0$$ $$60$$ $$5.0$$ $$70$$ $$4.3$$
##### 23.
 Temperature ($$T$$) Humidity ($$h$$) Jan. 1 $$\hphantom{000}34\degree$$F $$42\%$$ Jan. 2 $$\hphantom{000}36\degree$$F $$44\%$$ Jan. 3 $$\hphantom{000}35\degree$$F $$47\%$$ Jan. 4 $$\hphantom{000}29\degree$$F $$50\%$$ Jan. 5 $$\hphantom{000}31\degree$$F $$52\%$$ Jan. 6 $$\hphantom{000}35\degree$$F $$51\%$$ Jan. 7 $$\hphantom{000}34\degree$$F $$49\%$$
##### 24.
 Inflationrate ($$I$$) Unemploymentrate ($$U$$) 1972 $$\hphantom{000}5.6\%$$ $$5.1\%$$ 1973 $$\hphantom{000}6.2\%$$ $$4.5\%$$ 1974 $$\hphantom{000}10.1\%$$ $$4.9\%$$ 1975 $$\hphantom{000}9.2\%$$ $$7.4\%$$ 1976 $$\hphantom{000}5.8\%$$ $$6.7\%$$ 1977 $$\hphantom{000}5.6\%$$ $$6.8\%$$ 1978 $$\hphantom{000}6.7\%$$ $$7.4\%$$
##### 40.
The graph shows the U.S. unemployment rate, $$U\text{,}$$ as a function of time, $$t\text{,}$$ for the years 1985–2004. (Source: U.S. Bureau of Labor Statistics)
1. When did the unemployment rate reach its highest value, and what was its highest value?
2. When did the unemployment rate fall to its lowest value, and what was its lowest value?
3. Give two years in which the unemployment rate was $$4.5\%\text{.}$$

#### Exercise Group.

In Problems 41–48, evaluate each function for the given values.
##### 41.
$$f (x) = 6 - 2x$$
1. $$\displaystyle f(3)$$
2. $$\displaystyle f(-2)$$
3. $$\displaystyle f(12.7)$$
4. $$\displaystyle f\left(\dfrac{2}{3}\right)$$
##### 42.
$$g(t) = 5t - 3$$
1. $$\displaystyle g(1)$$
2. $$\displaystyle g(-4)$$
3. $$\displaystyle g(14.1)$$
4. $$\displaystyle g\left(\dfrac{3}{4}\right)$$
##### 43.
$$h(v) = 2v^2 - 3v + 1$$
1. $$\displaystyle h(0)$$
2. $$\displaystyle h(-1)$$
3. $$\displaystyle h\left(\dfrac{1}{4}\right)$$
4. $$\displaystyle h(-6.2)$$
##### 44.
$$r (s) = 2s - s^2$$
1. $$\displaystyle r(2)$$
2. $$\displaystyle r(-4)$$
3. $$\displaystyle r\left(\dfrac{1}{3}\right)$$
4. $$\displaystyle r(-1.3)$$
##### 45.
$$H(z) = \dfrac{2z - 3}{z + 2}$$
1. $$\displaystyle H(4)$$
2. $$\displaystyle H(-3)$$
3. $$\displaystyle H\left(\dfrac{4}{3}\right)$$
4. $$\displaystyle H(4.5)$$
##### 46.
$$F(x) = \dfrac{1-x}{2x-3}$$
1. $$\displaystyle F(0)$$
2. $$\displaystyle F(-3)$$
3. $$\displaystyle F\left(\dfrac{5}{2}\right)$$
4. $$\displaystyle F(9.8)$$
##### 47.
$$E(t) =\sqrt{t-4}$$
1. $$\displaystyle E(16)$$
2. $$\displaystyle E(4)$$
3. $$\displaystyle E(7)$$
4. $$\displaystyle E(4.2)$$
##### 48.
$$D(r) =\sqrt{5-r}$$
1. $$\displaystyle D(4)$$
2. $$\displaystyle D(-3)$$
3. $$\displaystyle D(-9)$$
4. $$\displaystyle D(4.6)$$

#### 49.

A sport utility vehicle costs \$$$28,000$$ and depreciates according to the formula
\begin{equation*} V(t) = 28,000 (1 - 0.08t) \end{equation*}
where $$V$$ is the value of the vehicle after $$t$$ years.
1. Evaluate $$V(12)$$ and explain what it means.
2. Solve the equation $$V(t) = 0$$ and explain what it means.
3. If this year is $$t = n\text{,}$$ what does $$V(n + 2)$$ mean?

#### 50.

In a profit-sharing plan, an employee receives a salary of
\begin{equation*} S(x) = 20,000 + 0.01x \end{equation*}
where $$x$$ represents the company’s profit for the year.
1. Evaluate $$S(850,000)$$ and explain what it means.
2. Solve the equation $$S(x) = 30,000$$ and explain what it means.
3. If the company made a profit of $$p$$ dollars this year, what does $$S(2p)$$ mean?

#### 51.

The number of compact cars that a large dealership can sell at price $$p$$ is given by
\begin{equation*} N( p) = \dfrac{12,000,000}{p} \end{equation*}
1. Evaluate $$N(6000)$$ and explain what it means.
2. As $$p$$ increases, does $$N(p)$$ increase or decrease? Why is this reasonable?
3. If the current price for a compact car is $$D\text{,}$$ what does $$2N(D)$$ mean?

#### 52.

A department store finds that the market value of its Christmas-related merchandise is given by
\begin{equation*} M(t) = \dfrac{600,000}{t},~~ t\le 30 \end{equation*}
where $$t$$ is the number of weeks after Christmas.
1. Evaluate $$M(2)$$ and explain what it means.
2. As $$t$$ increases, does $$M(t)$$ increase or decrease? Why is this reasonable?
3. If this week $$t = n\text{,}$$ what does $$M(n + 1)$$ mean?

#### 53.

The velocity of a car that brakes suddenly can be determined from the length of its skid marks, $$d\text{,}$$ by
\begin{equation*} v(d) = \sqrt{12d} \end{equation*}
where $$d$$ is in feet and $$v$$ is in miles per hour.
1. Evaluate $$v(250)$$ and explain what it means.
2. Estimate the length of the skid marks left by a car traveling at $$100$$ miles per hour.

#### 54.

The distance, $$d\text{,}$$ in miles that a person can see on a clear day from a height, $$h\text{,}$$ in feet is given by
\begin{equation*} d(h) = 1.22\sqrt{h} \end{equation*}
1. Evaluate $$d(20,320)$$ and explain what it means.
2. Estimate the height you need in order to see $$100$$ miles.

#### 55.

The figure gives data about snowfall, air temperature, and number of avalanches on the Mikka glacier in Sarek, Lapland, in 1957. (Source: Leopold, Wolman, Miller, 1992)
1. During June and July, avalanches occurred over three separate time intervals. What were they?
2. Over what three time intervals did snow fall?
3. When was the temperature above freezing ($$0\degree$$C)?

#### 56.

The bar graph shows the percent of Earth’s surface that lies at various altitudes or depths below the surface of the oceans. (Depths are given as negative altitudes.) (Source: Open University)
1. Read the graph and complete the table.
 Altitude (km) Percent ofEarth’s surface $$-7$$ to $$-6$$  $$-6$$ to $$-5$$  $$-5$$ to $$-4$$  $$-4$$ to $$-3$$  $$-3$$ to $$-2$$  $$-2$$ to $$-1$$  $$-1$$ to $$0$$  $$0$$ to $$1$$  $$1$$ to $$2$$  $$2$$ to $$3$$  $$3$$ to $$4$$  $$4$$ to $$5$$ 
2. What is the most common altitude? What is the second most common altitude??
3. Approximately what percent of the Earth’s surface is below sea level?
4. The height of Mt. Everest is $$8.85$$ kilometers. Can you think of a reason why it is not included in the graph?

#### 57.

The graph shows the temperature of the ocean at various depths. (Source: Open University)
1. Is depth a function of temperature?
2. Is temperature a function of depth?
3. The axes are scaled in an unusual way. Why is it useful to present the graph in this way?

#### 58.

The graph shows the relationship between annual precipitation, $$p\text{,}$$ in a region and the amount of erosion, measured in tons per square mile, $$s\text{.}$$ (Source: Leopold, Wolman, Miller, 1992)
1. Is the amount of erosion a function of the amount of precipitation?
2. At what annual precipitation is erosion at a maximum, and what is that maximum?
3. Over what interval of annual precipitation does erosion decrease?
4. An increase in vegetation inhibits erosion, and precipitation encourages vegetation. What happens to the amount of erosion as precipitation increases in each of these three environments?
 desert shrub: $$0\lt p\lt 12$$ grassland: $$12\lt p\lt 30$$ forest: $$30\lt p\lt 60$$

#### Exercise Group.

In Problems 59—64, evaluate the function and simplify.
##### 59.
$$G(s) = 3s^2 - 6s$$
1. $$\displaystyle G(3a)$$
2. $$\displaystyle G(a + 2)$$
3. $$\displaystyle G(a) + 2$$
4. $$\displaystyle G(-a)$$
##### 60.
$$h(x) = 2x^2 + 6x - 3$$
1. $$\displaystyle h(2a)$$
2. $$\displaystyle h(a + 3)$$
3. $$\displaystyle h(a) + 3$$
4. $$\displaystyle h(-a)$$
##### 61.
$$g(x) = 8$$
1. $$\displaystyle g(2)$$
2. $$\displaystyle g(8)$$
3. $$\displaystyle g(a + 1)$$
4. $$\displaystyle g(-x)$$
##### 62.
$$f (t) = -3$$
1. $$\displaystyle f (4)$$
2. $$\displaystyle f (-3)$$
3. $$\displaystyle f (b - 2)$$
4. $$\displaystyle f (-t)$$
##### 63.
$$P(x) = x^3 - 1$$
1. $$\displaystyle P(2x)$$
2. $$\displaystyle 2P(x)$$
3. $$\displaystyle P(x^2)$$
4. $$\displaystyle [P(x)]^2$$
##### 64.
$$Q(t) = 5t^3$$
1. $$\displaystyle Q(2t)$$
2. $$\displaystyle 2Q(t)$$
3. $$\displaystyle Q(t^2)$$
4. $$\displaystyle [Q(t)]^2$$

#### Exercise Group.

In Problems 65—68, evaluate the function for the given expressions and simplify.
##### 65.
$$f (x) = x^3$$
1. $$\displaystyle f (a^2)$$
2. $$\displaystyle a^3 \cdot f (a^3)$$
3. $$\displaystyle f (ab)$$
4. $$\displaystyle f (a + b)$$
##### 66.
$$g(x) = x^4$$
1. $$\displaystyle g(a^3)$$
2. $$\displaystyle a^4\cdot g(a^4)$$
3. $$\displaystyle g(ab)$$
4. $$\displaystyle g(a + b)$$
##### 67.
$$F(x) = 3x^5$$
1. $$\displaystyle F(2a)$$
2. $$\displaystyle 2 F(a)$$
3. $$\displaystyle F(a^2)$$
4. $$\displaystyle [F(a)]^2$$
##### 68.
$$G(x) = 4x^3$$
1. $$\displaystyle G(3a)$$
2. $$\displaystyle 3G(a)$$
3. $$\displaystyle G(a^4)$$
4. $$\displaystyle [G(a)]^4$$

#### Exercise Group.

For the functions in Problems 69–76, compute the following:
1. $$\displaystyle f (2) + f (3)$$
2. $$\displaystyle f (2 + 3)$$
3. $$\displaystyle f (a) + f (b)$$
4. $$\displaystyle f (a + b)$$
For which functions does $$f (a + b) = f (a) + f (b)$$ for all values of $$a$$ and $$b\text{?}$$
##### 69.
$$f (x) = 3x - 2$$
##### 70.
$$f (x) = 1 - 4x$$
##### 71.
$$f (x) = x^2 + 3$$
##### 72.
$$f (x) = x^2 - 1$$
##### 73.
$$f (x) =\sqrt{x+1}$$
##### 74.
$$f (x) = \sqrt{6-x}$$
##### 75.
$$f (x) =\dfrac{-2}{x}$$
##### 76.
$$f (x) = \dfrac{3}{x}$$

#### 77.

Use a table of values to estimate a solution to
\begin{equation*} f (x) = 800 + 6x - 0.2x^2 = 500 \end{equation*}
as follows:
1. Make a table starting at $$x = 0$$ and increasing by $$\Delta x = 10\text{,}$$ as shown in the accompanying tables. Find two $$x$$-values $$a$$ and $$b$$ so that $$f (a)\gt 500\gt f (b)\text{.}$$
 $$x$$ $$0$$ $$10$$ $$20$$ $$30$$ $$40$$ $$50$$ $$60$$ $$70$$ $$80$$ $$90$$ $$100$$ $$f(x)$$           
2. Make a new table starting at $$x = a$$ and increasing by $$\Delta x = 1\text{.}$$ Find two $$x$$-values, $$c$$ and $$d\text{,}$$ so that $$f (c)\gt 500\gt f (d)\text{.}$$
3. Make a new table starting at $$x = c$$ and increasing by $$\Delta x = 0.1\text{.}$$ Find two $$x$$-values, $$p$$ and $$q\text{,}$$ so that $$f (p)\gt 500\gt f (q)\text{.}$$
4. Take the average of $$p$$ and $$q\text{,}$$ that is, set $$s = \dfrac{p + q}{2}\text{.}$$ Then $$s$$ is an approximate solution that is off by at most $$0.05\text{.}$$
5. Evaluate $$f (s)$$ to check that the output is approximately $$500\text{.}$$

#### 78.

Use a table of values to estimate a solution to
\begin{equation*} f (x) = x^3 - 4x^2 + 5x = 18, 000 \end{equation*}
as follows:
1. Make a table starting at $$x = 0$$ and increasing by $$\Delta x = 10\text{,}$$ as shown in the accompanying tables. Find two $$x$$-values $$a$$ and $$b$$ so that $$f (a)\lt 18,000\lt f (b)\text{.}$$
 $$x$$ $$0$$ $$10$$ $$20$$ $$30$$ $$40$$ $$50$$ $$60$$ $$70$$ $$80$$ $$90$$ $$100$$ $$f(x)$$           
2. Make a new table starting at $$x = a$$ and increasing by $$\Delta x = 1\text{.}$$ Find two $$x$$-values, $$c$$ and $$d\text{,}$$ so that $$f (c)\lt 18,000\lt f (d)\text{.}$$
3. Make a new table starting at $$x = c$$ and increasing by $$\Delta x = 0.1\text{.}$$ Find two $$x$$-values, $$p$$ and $$q\text{,}$$ so that $$f (p)\lt 18,000\lt f (q)\text{.}$$
4. Take the average of $$p$$ and $$q\text{,}$$ that is, set $$s = \dfrac{p + q}{2}\text{.}$$ Then $$s$$ is an approximate solution that is off by at most $$0.05\text{.}$$
5. Evaluate $$f (s)$$ to check that the output is approximately $$18,000\text{.}$$

#### 79.

Use tables of values to estimate the positive solution to
\begin{equation*} f (x) = x^2 - \dfrac{1}{x} = 9000\text{,} \end{equation*}
accurate to within $$0.05\text{.}$$

#### 80.

Use tables of values to estimate the positive solution to
\begin{equation*} f (x) = \dfrac{8}{x}+500-\dfrac{x^2}{9} = 300\text{,} \end{equation*}
accurate to within $$0.05\text{.}$$