SECTION 2.6 Mathematical Models: Building Functions 117
Figure 59
x y
⫺1 2
(0, 0) P⫽ (x, y) y ⫽ x2 ⫺ 1
⫺1 1 2
1 d
0 2 0 2 Figure 60
language of mathematics. We do this by assigning symbols to represent the independent and dependent variables and then by finding the function or rule that relates these variables.
Finding the Distance from the Origin to a Point on a Graph Let P = 1x, y2 be a point on the graph of y= x2 -1.
(a) Express the distance d from P to the origin O as a function of x.
(b) What is d if x= 0?
(c) What is d if x= 1?
(d) What is d if x= 22 2 ?
(e) Use a graphing utility to graph the function d =d1x2, x Ú0. Rounded to two decimal places, find the value(s) of x at which d has a local minimum. [This gives the point(s) on the graph of y= x2 -1 closest to the origin.]
(a) Figure 59 illustrates the graph of y= x2- 1. The distance d from P to O is d= 21x- 022+ 1y -022 = 2x2+ y2
Since P is a point on the graph of y= x2- 1, substitute x2 -1 for y. Then d1x2 = 2x2+ 1x2- 122 = 2x4- x2 +1
The distance d is expressed as a function of x.
(b) If x= 0, the distance d is
d102 = 204- 02 +1 = 21 =1 (c) If x= 1, the distance d is
d112 = 214- 12 +1 =1 (d) If x = 22
2 , the distance d is da22
2 b = Ba
12
2 b4- a22
2 b2 +1 = B
1 4 - 1
2 + 1= 13 2
(e) Figure 60 shows the graph of Y1= 2x4- x2 +1 . Using the MINIMUM feature on a graphing utility, we find that when x ⬇ 0.71 the value of d is smallest. The local minimum is d ⬇ 0.87 rounded to two decimal places. Since d1x2 is even, by symmetry, it follows that when x ⬇ -0.71 the value of d is also a local minimum. Since 1{0.7122 -1 ⬇ -0.50, the points 1-0.71, -0.502 and
10.71, -0.502 on the graph of y= x2 -1 are closest to the origin.
Now Work P R O B L E M 1
Area of a Rectangle
A rectangle has one corner in quadrant I on the graph of y =25 -x2, another at the origin, a third on the positive y-axis, and the fourth on the positive x-axis. See Figure 61 on the next page.
(a) Express the area A of the rectangle as a function of x.
(b) What is the domain of A?
(c) Graph A=A1x2.
(d) For what value of x is the area largest?
E X A M P L E 1
Solution
E X A M P L E 2
(a) The area A of the rectangle is A =xy, where y= 25- x2. Substituting this expression for y, we obtain A1x2 = x125- x22 = 25x -x3.
(b) Since 1x, y2 is in quadrant I, we have x 70. Also, y =25- x2 70, which implies that x26 25, so -56 x6 5. Combining these restrictions, we have the domain of A as 5 x 兩 06 x6 5 6 , or 10, 52 using interval notation.
(c) See Figure 62 for the graph of A= A1x2.
(d) Using MAXIMUM, we find that the maximum area is 48.11 square units at x =2.89 units, each rounded to two decimal places. See Figure 63.
Solution
Now Work P R O B L E M 7
Close Call ?
Suppose two planes flying at the same altitude are headed toward each other. One plane is flying due South at a groundspeed of 400 miles per hour and is 600 miles from the potential intersection point of the planes. The other plane is flying due West with a groundspeed of 250 miles per hour and is 400 miles from the potential intersection point of the planes. See Figure 64.
(a) Build a model that expresses the distance d between the planes as a function of time t.
(b) Use a graphing utility to graph d =d1t2. How close do the planes come to each other? At what time are the planes closest?
(a) Refer to Figure 64. The distance d between the two planes is the hypotenuse of a right triangle. At any time t the length of the North/South leg of the triangle is 600-400t. At any time t, the length of the East/West leg of the triangle is 400-250t. Using the Pythagorean Theorem, the square of the distance between the two planes is
d2 = 1600 -400t22 + 1400- 250t22
Therefore, the distance between the two planes as a function of time is given by the model
d1t2 = 21600 -400t22 + 1400 -250t22
(b) Figure 65(a) shows the graph of d= d1t2. Using MINIMUM, the minimum distance between the planes is 21.20 miles and the time at which the planes are closest is after 1.53 hours, each rounded to two decimal places. See Figure 65(b).
E X A M P L E 3
Solution
Figure 62 50
0
0 5
Figure 63 50
0
0 5
E N
400 mph
d Plane
Plane 250 mph 400 miles
600 miles Figure 64
Figure 65 500
⫺50
0 2
(a) (b)
Figure 61
x y
10 20 30
⫺1
(x, y)
y ⫽ 25 ⫺ x2
5 4 1 2 3 (0,0)
SECTION 2.6 Mathematical Models: Building Functions 119 2.6 Assess Your Understanding
8. A rectangle is inscribed in a semicircle of radius 2. See the figure. Let P= 1x, y2 be the point in quadrant I that is a vertex of the rectangle and is on the circle.
Applications and Extensions
x y y ⫽ x 3
(x, y) (0, y)
(0, 0)
x y
4 (x, y) y ⫽ 16 ⫺ x 2
(0,0) 8 16
(a) Express the area A of the rectangle as a function of x.
(b) Express the perimeter p of the rectangle as a function of x.
(c) Graph A= A1x2. For what value of x is A largest?
(d) Graph p= p1x2. For what value of x is p largest?
9. A rectangle is inscribed in a circle of radius 2. See the figure.
Let P= 1x, y2 be the point in quadrant I that is a vertex of the rectangle and is on the circle.
x y
2 P ⫽ (x, y) 4 ⫺ x 2
y ⫽
⫺2
(a) Express the area A of the rectangle as a function of x.
(b) Express the perimeter p of the rectangle as a function of x.
(c) Graph A= A1x2. For what value of x is A largest?
(d) Graph p=p1x2. For what value of x is p largest?
10. A circle of radius r is inscribed in a square. See the figure.
1. Let P= 1x, y2 be a point on the graph of y= x2 -8.
(a) Express the distance d from P to the origin as a function of x.
(b) What is d if x=0?
(c) What is d if x=1?
(d) Use a graphing utility to graph d= d1x2.
(e) For what values of x is d smallest?
2. Let P= 1x, y2 be a point on the graph of y= x2 -8.
(a) Express the distance d from P to the point 10, -12 as a function of x.
(b) What is d if x=0?
(c) What is d if x= -1?
(d) Use a graphing utility to graph d= d1x2.
(e) For what values of x is d smallest?
3. Let P= 1x, y2 be a point on the graph of y= 1x. (a) Express the distance d from P to the point 11, 02 as a
function of x.
(b) Use a graphing utility to graph d= d1x2.
(c) For what values of x is d smallest?
4. Let P= 1x, y2 be a point on the graph of y= 1 x.
(a) Express the distance d from P to the origin as a function of x.
(b) Use a graphing utility to graph d= d1x2.
(c) For what values of x is d smallest?
5. A right triangle has one vertex on the graph of y= x3, x70, at 1x, y2, another at the origin, and the third on the positive y-axis at 10, y2, as shown in the figure. Express the area A of the triangle as a function of x.
6. A right triangle has one vertex on the graph of y=9- x2, x70, at 1x, y2, another at the origin, and the third on the positive x-axis at 1x, 02. Express the area A of the triangle as a function of x.
7. A rectangle has one corner in quadrant I on the graph of y=16 -x2, another at the origin, a third on the positive y-axis, and the fourth on the positive x-axis. See the figure.
(a) Express the area A of the rectangle as a function of x.
(b) What is the domain of A?
(c) Graph A=A1x2. For what value of x is A largest?
x y
2 P ⫽ (x, y)
x 2⫹ y 2⫽ 4
⫺2 2
⫺2
r
(a) Express the area A of the square as a function of the radius r of the circle.
(b) Express the perimeter p of the square as a function of r.
(a) Express the total area A enclosed by the pieces of wire as a function of the length x of a side of the square.
(b) What is the domain of A?
(c) Graph A=A1x2. For what value of x is A smallest?
12. Geometry A wire 10 meters long is to be cut into two pieces. One piece will be shaped as an equilateral triangle, and the other piece will be shaped as a circle.
(a) Express the total area A enclosed by the pieces of wire as a function of the length x of a side of the equilateral triangle.
(b) What is the domain of A?
(c) Graph A=A1x2. For what value of x is A smallest?
13. A wire of length x is bent into the shape of a circle.
(a) Express the circumference C of the circle as a function of x.
(b) Express the area A of the circle as a function of x.
14. A wire of length x is bent into the shape of a square.
(a) Express the perimeter p of the square as a function of x.
(b) Express the area A of the square as a function of x.
15. Geometry A semicircle of radius r is inscribed in a rectangle so that the diameter of the semicircle is the length of the rectangle. See the figure.
(a) Express the area A of the rectangle as a function of the radius r of the semicircle.
(b) Express the perimeter p of the rectangle as a function of r.
16. Geometry An equilateral triangle is inscribed in a circle of radius r. See the figure. Express the circumference C of the circle as a function of the length x of a side of the triangle.
[Hint: First show that r2= x2 3.]
17. Geometry An equilateral triangle is inscribed in a circle of radius r. See the figure in Problem 16. Express the area A within the circle, but outside the triangle, as a function of the length x of a side of the triangle.
r
x x
x
r 21. Inscribing a Cylinder in a Cone Inscribe a right circular
cylinder of height h and radius r in a cone of fixed radius R and fixed height H. See the illustration. Express the volume V of the cylinder as a function of r.
[Hint: V=pr2h. Note also the similar triangles.]
d N
S E W
Sphere r
R h
4x x
10 ⫺ 4x 10 m
19. Uniform Motion Two cars are approaching an intersection.
One is 2 miles south of the intersection and is moving at a constant speed of 30 miles per hour. At the same time, the other car is 3 miles east of the intersection and is moving at a constant speed of 40 miles per hour.
(a) Build a model that expresses the distance d between the cars as a function of time t.
[Hint: At t= 0, the cars are 2 miles south and 3 miles east of the intersection, respectively.]
(b) Use a graphing utility to graph d=d1t2. For what value of t is d smallest?
20. Inscribing a Cylinder in a Sphere Inscribe a right circular cylinder of height h and radius r in a sphere of fixed radius R. See the illustration. Express the volume V of the cylinder as a function of h.
[Hint: V=pr2h. Note also the right triangle.]
11. Geometry A wire 10 meters long is to be cut into two pieces. One piece will be shaped as a square, and the other piece will be shaped as a circle. See the figure.
18. Uniform Motion Two cars leave an intersection at the same time. One is headed south at a constant speed of 30 miles per hour, and the other is headed west at a constant speed of 40 miles per hour (see the figure). Build a model that expresses the distance d between the cars as a function of the time t.
[Hint: At t=0, the cars leave the intersection.]
SECTION 2.6 Mathematical Models: Building Functions 121
22. Installing Cable TV MetroMedia Cable is asked to provide service to a customer whose house is located 2 miles from the road along which the cable is buried. The nearest connection box for the cable is located 5 miles down the road. See the figure.
(a) If the installation cost is $500 per mile along the road and $700 per mile off the road, build a model that expresses the total cost C of installation as a function of the distance x (in miles) from the connection box to the point where the cable installation turns off the road.
Give the domain.
(b) Compute the cost if x =1 mile.
(c) Compute the cost if x =3 miles.
(d) Graph the function C=C1x2. Use TRACE to see how the cost C varies as x changes from 0 to 5.
(e) What value of x results in the least cost?
23. Time Required to Go from an Island to a Town An island is 2 miles from the nearest point P on a straight shoreline. A town is 12 miles down the shore from P. See the illustration.
Cone r
R H
h
Box Stream
House
5 mi 2 mi
x
per hour, build a model that expresses the time T that it takes to go from the island to town as a function of the distance x from P to where the person lands the boat.
(b) What is the domain of T ?
(c) How long will it take to travel from the island to town if the person lands the boat 4 miles from P?
(d) How long will it take if the person lands the boat 8 miles from P ?
24. Filling a Conical Tank Water is poured into a container in the shape of a right circular cone with radius 4 feet and height 16 feet. See the figure. Express the volume V of the water in the cone as a function of the height h of the water.
[Hint: The volume V of a cone of radius r and height h is V= 1
3 pr2h.]
12 mi
2 mi
12 ⫺ x x
d2 P
d1
Island
Town
25. Constructing an Open Box An open box with a square base is to be made from a square piece of cardboard 24 inches on a side by cutting out a square from each corner and turning up the sides. See the figure.
h 16
4 r
24 in.
24 in.
x x
x x
x x
x x
(a) Express the volume V of the box as a function of the length x of the side of the square cut from each corner.
(b) What is the volume if a 3-inch square is cut out?
(c) What is the volume if a 10-inch square is cut out?
(d) Graph V= V1x2. For what value of x is V largest?
26. Constructing an Open Box An open box with a square base is required to have a volume of 10 cubic feet.
(a) Express the amount A of material used to make such a box as a function of the length x of a side of the square base.
(b) How much material is required for a base 1 foot by 1 foot?
(c) How much material is required for a base 2 feet by 2 feet?
(d) Use a graphing utility to graph A= A1x2. For what value of x is A smallest?
(a) If a person can row a boat at an average speed of 3 miles per hour and the same person can walk 5 miles
CHAPTER REVIEW
x y
f(x) = b (0,b)
x y
3 3
– 3
(1, 1)
( – 1, – 1) (0, 0)
x y
4 4
– 4
(2, 4)
(0, 0) ( – 2, 4)
(1, 1) (– 1, 1)
x y
4 4
⫺4
(1, 1) (0, 0) (⫺1, ⫺1)
⫺4
x y
5 2
⫺1 (1, 1)
(0, 0)
(4, 2)
( , )
x y
3 (1, 1)
(⫺1, ⫺1) (2, 2 )
(0, 0)
⫺3
⫺3
3
3
(⫺2,⫺ 2 )3
1– 8 1– 2
(⫺ ,1–8⫺ 1–2)
x y
2 2
(1, 1)
(⫺1, ⫺1)
⫺2
⫺2 x
y
3 3
⫺3
(1, 1) (0, 0) (⫺1, 1)
(2, 2) (⫺2, 2)
x y
4 2
⫺2 2 4
⫺3
Library of Functions
Constant function (p. 95) f1x2 =b
The graph is a horizontal line with y-intercept b.
Identity function (p. 95) f1x2 = x
The graph is a line with slope 1 and y-intercept 0.
Square function (p. 96) f1x2 = x2
The graph is a parabola with intercept at (0, 0).
Cube function (p. 96) f1x2 =x3
Square root function (pp. 93 and 96) f1x2 = 1x
Cube root function (pp. 94 and 96) f1x2 = 13x
Reciprocal function (p. 96) f1x2 =1
x
Absolute value function (p. 97) f1x2 = 0x0
Greatest integer function (p. 97) f1x2 = int1x2
Things to Know
Function (pp. 58 – 61) A relation between two sets so that each element x in the first set, the domain, has corresponding to it exactly one element y in the second set. The range is the set of y values of the function for the x values in the domain.
A function can also be characterized as a set of ordered pairs (x, y) in which no first element is paired with two different second elements.
Function notation (pp. 61– 64) y= f1x2
f is a symbol for the function.
x is the argument, or independent variable.
y is the dependent variable.
f1x2 is the value of the function at x, or the image of x.
A function f may be defined implicitly by an equation involving x and y or explicitly by writing y =f1x2.
Difference quotient of f (pp. 63 and 92) f1x+h2 -f1x2
h h⬆0
Chapter Review 123
Domain (pp. 64–65) If unspecified, the domain of a function f defined by an equation is the largest set of real numbers for which f1x2 is a real number.
Vertical-line test (p. 72) A set of points in the plane is the graph of a function if and only if every vertical line intersects the graph in at most one point.
Even function f (p. 81) f1-x2 = f1x2 for every x in the domain (-x must also be in the domain).
Odd function f (p. 81) f1-x2 = -f1x2 for every x in the domain (-x must also be in the domain).
Increasing function (p. 83) A function f is increasing on an open interval I if, for any choice of x1 and x2 in I, with x1 6x2, we have f1x12 6f1x22.
Decreasing function (p. 83) A function f is decreasing on an open interval I if, for any choice of x1 and x2 in I, with x1 6x2, we have f1x12 7f1x22.
Constant function (p. 84) A function f is constant on an open interval I if, for all choices of x in I, the values of f1x2are equal.
Local maximum (p. 84) A function f has a local maximum at c if there is an open interval I containing c so that, for all x in I, f1x2 …f1c2.
Local minimum (p. 84) A function f has a local minimum at c if there is an open interval I containing c so that, for all x in I, f1x2 Úf1c2.
Absolute maximum and Absolute minimum (p. 85)
Let f denote a function defined on some interval I.
If there is a number u in I for which f1x2 …f1u2 for all x in I, then f1u2 is the absolute maximum of f on I and we say the absolute maximum of f occurs at u.
If there is a number v in I for which f1x2 Úf1v2, for all x in I, then f1v2 is the absolute minimum of f on I and we say the absolute minimum of f occurs at v.
Average rate of change of a function (p. 87) The average rate of change of f from a to b is y
x = f1b2 -f1a2 b-a a⬆b
Section You should be able to . . . Examples Review Exercises 2.1 1 Determine whether a relation represents a function (p. 58) 1–5 1, 2
2 Find the value of a function (p. 61) 6, 7 3–5, 15, 39
3 Find the domain of a function defined by an equation (p. 64) 8, 9 6–11 4 Form the sum, difference, product, and quotient of two functions (p. 66) 10 12–14
2.2 1 Identify the graph of a function (p. 72) 1 27, 28
2 Obtain information from or about the graph of a function (p. 73) 2–4 16(a) – (e), 17(a),
17(e), 17(g)
2.3 1 Determine even and odd functions from a graph (p. 81) 1 17(f)
2 Identify even and odd functions from the equation (p. 82) 2 18 – 21 3 Use a graph to determine where a function is increasing, decreasing,
or constant (p. 83) 3 17(b)
4 Use a graph to locate local maxima and local minima (p. 84) 4 17(c) 5 Use a graph to locate the absolute maximum and the absolute
minimum (p. 85) 5 17(d)
6 Use a graphing utility to approximate local maxima and local minima
and to determine where a function is increasing or decreasing (p. 86) 6 22, 23, 40(d), 41(b) 7 Find the average rate of change of a function (p. 87) 7, 8 24–26
2.4 1 Graph the functions listed in the library of functions (p. 93) 1, 2 29, 30
2 Graph piecewise-defined functions (p. 98) 3, 4 37, 38
2.5 1 Graph functions using vertical and horizontal shifts (p. 104) 1, 2, 5, 6 16(f), 31, 33, 34,
35, 36
2 Graph functions using compressions and stretches (p. 106) 3, 5, 6 16(g), 32, 36 3 Graph functions using reflections about the x-axis or y-axis (p. 108) 6 16(h), 32, 34, 36
2.6 1 Build and analyze functions (p. 116) 1–3 40, 41
Objectives
Review Exercises
In Problems 1 and 2, determine whether each relation represents a function. For each function, state the domain and range.
1. 5(-1, 0), (2, 3), (4, 0)6 2. 5(4, -1), (2, 1), (4, 2)6 In Problems 3–5, find the following for each function:
(a) f122 (b) f1-22 (c) f1-x2 (d) -f1x2 (e) f1x-22 (f) f12x2 3. f1x2 = 3x
x2-1 4. f1x2 = 2x2- 4 5. f1x2 = x2-4
x2 In Problems 6–11, find the domain of each function.
6. f1x2 = x
x2-9 7. f1x2 = 22 -x 8. g(x)= 0x0
x 9. f1x2 = x
x2+2x -3 10. f1x2 = 1x+ 1
x2- 4 11. g(x)= x
1x +8 In Problems 12–14, find f+g, f-g, f#g, and f
g for each pair of functions. State the domain of each of these functions.
12. f1x2 =2 -x; g(x)=3x +1 13. f1x2 =3x2+ x+1; g(x)=3x 14. f1x2 = x+ 1
x- 1; g(x)= 1 x 15. Find the difference quotient of f1x2 = -2x2+ x+1; that is, find f1x+h2 -f1x2
h , h⬆0.
16. Use the graph of the function f shown to find:
(a) Find the domain and the range of f.
(b) List the intercepts.
(c) Find f1-22.
(d) Find the value(s) of x for which f1x2 = -3.
(e) Solve f1x2 70.
(f) Graph y=f1x-32.
(g) Graph y=fa1 2 xb. (h) Graph y= -f1x2.
(3, 3)
(⫺2, ⫺1) (⫺4, ⫺3)
x y
⫺5 5
4
⫺4 (0, 0)
17. Use the graph of the function f shown to find:
(a) The domain and the range of f .
(b) The intervals on which f is increasing, decreasing, or constant.
(c) The local minimum values and local maximum values.
(d) The absolute maximum and absolute minimum.
(e) Whether the graph is symmetric with respect to the x-axis, the y-axis, or the origin.
(f) Whether the function is even, odd, or neither.
(g) The intercepts, if any.
x y
⫺6 6
4
⫺4
(4, 3)
(⫺4,⫺3) (2, ⫺1) (3, 0) (⫺2, 1)
(⫺3, 0)
In Problems 18–21, determine (algebraically) whether the given function is even, odd, or neither.
18. f1x2 =x3- 4x 19. g(x)= 4+x2
1+x4 20. G(x)= 1-x+ x3 21. f1x2 = x 1+ x2 In Problems 22 and 23, use a graphing utility to graph each function over the indicated interval. Approximate any local maximum values and local minimum values. Determine where the function is increasing and where it is decreasing.
22. f1x2 =2x3- 5x+ 1 (-3, 3) 23. f1x2 =2x4- 5x3 +2x+ 1 (-2, 3) 24. Find the average rate of change of f1x2 =8x2-x.
(a) From 1 to 2 (b) From 0 to 1 (c) From 2 to 4
In Problems 25 and 26, find the average rate of change from 2 to 3 for each function f. Be sure to simplify.
25. f1x2 =2 -5x 26. f1x2 =3x -4x2
Chapter Test 125
In Problems 29 and 30, sketch the graph of each function. Be sure to label at least three points.
29. f1x2 = 0x0 30. f1x2 = 1x
In Problems 31–36, graph each function using the techniques of shifting, compressing or stretching, and reflections. Identify any intercepts on the graph. State the domain and, based on the graph, find the range.
31. F1x2 = 0x0 - 4 32. g1x2 = -20x0 33. h(x)= 2x- 1 34. f1x2 = 21- x 35. h(x)=(x- 1)2+2 36. g(x)= -2(x+ 2)3-8 In Problems 37 and 38,
(a) Find the domain of each function. (b) Locate any intercepts. (c) Graph each function.
(d) Based on the graph, find the range. (e) Is f continuous on its domain?
x y
x y
37. f1x2 = b3x if -26 x…1 x+1 if x7 1 38. f1x2 = c
x if -4…x6 0 1 if x=0 3x if x70
39. A function f is defined by f1x2 = Ax+5
6x-2 If f112 =4, find A.
40. Constructing a Closed Box A closed box with a square base is required to have a volume of 10 cubic feet.
(a) Build a model that expresses the amount A of material used to make such a box as a function of the length x of a side of the square base.
(b) How much material is required for a base 1 foot by 1 foot?
(c) How much material is required for a base 2 feet by 2 feet?
(d) Graph A=A(x). For what value of x is A smallest?
41. A rectangle has one vertex in quadrant I on the graph of y=10 -x2, another at the origin, one on the positive x-axis, and one on the positive y-axis.
(a) Express the area A of the rectangle as a function of x.
(b) Find the largest area A that can be enclosed by the rectangle.
CHAPTER TEST
1. Determine whether each relation represents a function. For each function, state the domain and the range.
(a) 5(2, 5), (4, 6), (6, 7), (8, 8)6 (b) 5(1, 3), (4, -2), (-3, 5), (1, 7)6 (c)
y 6 4 2
⫺2
⫺4
x 4
⫺4 ⫺2 2
The Chapter Test Prep Videos are step-by-step test solutions available in the Video Resources DVD, in , or on this text’s Channel. Flip back to the Student Resources page to see the exact web address for this text’s YouTube channel.
(d)
y 6 4 2
⫺2
x 4
⫺4 ⫺2 2 In Problems 27 and 28, is the graph shown the graph of a function?
27. 28.