🎉 Announcing Numerade's $26M Series A, led by IDG Capital!Read how Numerade will revolutionize STEM Learning # Calculus for Business, Economics, Life Sciences, and Social Sciences 13th ## Raymond A. Barnett, Michael R. Ziegler, Karl E. Byleen ## Chapter 7 ## Multivariable Calculus ## Educators ### Problem 1 Find the indicated value of the function of two or three variables. (If necessary, review Appendix$\mathrm{C}$). The height of a trapezoid is 3 feet and the lengths of its parallel sides are 5 feet and 8 feet. Find the area. Dwijendra R. Numerade Educator ### Problem 2 Find the indicated value of the function of two or three variables. (If necessary, review Appendix$\mathrm{C}$). The height of a trapezoid is 4 meters and the lengths of its parallel sides are 25 meters and 32 meters. Find the area. Dwijendra R. Numerade Educator ### Problem 3 Find the indicated value of the function of two or three variables. (If necessary, review Appendix$\mathrm{C}$). The length, width, and height of a rectangular box are 12 inches, 5 inches, and 4 inches, respectively. Find the volume. Dwijendra R. Numerade Educator ### Problem 4 Find the indicated value of the function of two or three variables. (If necessary, review Appendix$\mathrm{C}$). The length, width, and height of a rectangular box are 30 centimeters, 15 centimeters, and 10 centimeters, respectively. Find the volume. Dwijendra R. Numerade Educator ### Problem 5 Find the indicated value of the function of two or three variables. (If necessary, review Appendix$\mathrm{C}$). The height of a right circular cylinder is 8 meters and the radius is 2 meters. Find the volume. Dwijendra R. Numerade Educator ### Problem 6 Find the indicated value of the function of two or three variables. (If necessary, review Appendix$\mathrm{C}$). The height of a right circular cylinder is 6 feet and the diameter is also 6 feet. Find the total surface area. Dwijendra R. Numerade Educator ### Problem 7 Find the indicated value of the function of two or three variables. (If necessary, review Appendix$\mathrm{C}$). The height of a right circular cone is 48 centimeters and the radius is 20 centimeters. Find the total surface area. Dwijendra R. Numerade Educator ### Problem 8 Find the indicated value of the function of two or three variables. (If necessary, review Appendix$\mathrm{C}$). The height of a right circular cone is 42 inches and the radius is 7 inches. Find the volume. Dwijendra R. Numerade Educator ### Problem 9 Find the indicated values of the functions$f(x, y)=2 x+7 y-5 \quad \text { and } \quad g(x, y)=\frac{88}{x^{2}+3 y}$. $$f(4,-1)$$ Dwijendra R. Numerade Educator ### Problem 10 Find the indicated values of the functions$f(x, y)=2 x+7 y-5 \quad \text { and } \quad g(x, y)=\frac{88}{x^{2}+3 y}$. $$f(0,10)$$ Dwijendra R. Numerade Educator ### Problem 11 Find the indicated values of the functions$f(x, y)=2 x+7 y-5 \quad \text { and } \quad g(x, y)=\frac{88}{x^{2}+3 y}$. $$f(8,0)$$ Dwijendra R. Numerade Educator ### Problem 12 Find the indicated values of the functions$f(x, y)=2 x+7 y-5 \quad \text { and } \quad g(x, y)=\frac{88}{x^{2}+3 y}$. $$f(5,6)$$ Dwijendra R. Numerade Educator ### Problem 13 Find the indicated values of the functions$f(x, y)=2 x+7 y-5 \quad \text { and } \quad g(x, y)=\frac{88}{x^{2}+3 y}$. $$g(1,7)$$ Dwijendra R. Numerade Educator ### Problem 14 Find the indicated values of the functions$f(x, y)=2 x+7 y-5 \quad \text { and } \quad g(x, y)=\frac{88}{x^{2}+3 y}$. $$g(-2,0)$$ Dwijendra R. Numerade Educator ### Problem 15 Find the indicated values of the functions$f(x, y)=2 x+7 y-5 \quad \text { and } \quad g(x, y)=\frac{88}{x^{2}+3 y}$. $$g(3,-3)$$ Dwijendra R. Numerade Educator ### Problem 16 Find the indicated values of the functions$f(x, y)=2 x+7 y-5 \quad \text { and } \quad g(x, y)=\frac{88}{x^{2}+3 y}$. $$g(0,0)$$ Dwijendra R. Numerade Educator ### Problem 17 Find the indicated values of$f(x, y, z)=2 x-3 y^{2}+5 z^{3}-1$. $$f(0,0,0)$$ Dwijendra R. Numerade Educator ### Problem 18 Find the indicated values of$f(x, y, z)=2 x-3 y^{2}+5 z^{3}-1$. $$f(0,0,2)$$ Dwijendra R. Numerade Educator ### Problem 19 Find the indicated values of$f(x, y, z)=2 x-3 y^{2}+5 z^{3}-1$. $$f(6,-5,0)$$ Dwijendra R. Numerade Educator ### Problem 20 Find the indicated values of$f(x, y, z)=2 x-3 y^{2}+5 z^{3}-1$. $$f(-10,4,-3)$$ Dwijendra R. Numerade Educator ### Problem 21 Find the indicated value of the given function. $$P(13,5) \text { for } P(n, r)=\frac{n !}{(n-r) !}$$ Dwijendra R. Numerade Educator ### Problem 22 Find the indicated value of the given function. $$C(13,5) \text { for } C(n, r)=\frac{n !}{r !(n-r) !}$$ Dwijendra R. Numerade Educator ### Problem 23 Find the indicated value of the given function. $$V(4,12) \text { for } V(R, h)=\pi R^{2} h$$ Dwijendra R. Numerade Educator ### Problem 24 Find the indicated value of the given function. $$T(4,12) \text { for } T(R, h)=2 \pi R(R+h)$$ Dwijendra R. Numerade Educator ### Problem 25 Find the indicated value of the given function. $$S(3,10) \text { for } S(R, h)=\pi R \sqrt{R^{2}+h^{2}}$$ Dwijendra R. Numerade Educator ### Problem 26 Find the indicated value of the given function. $$W(3,10) \text { for } W(R, h)=\frac{1}{3} \pi R^{2} h$$ Dwijendra R. Numerade Educator ### Problem 27 Find the indicated value of the given function. $$A(100,0.06,3) \text { for } A(P, r, t)=P+P r t$$ Dwijendra R. Numerade Educator ### Problem 28 Find the indicated value of the given function. $$A(10,0.04,3,2) \text { for } A(P, r, t, n)=P\left(1+\frac{r}{n}\right)^{t n}$$ Dwijendra R. Numerade Educator ### Problem 29 Find the indicated value of the given function. $$P(0.05,12) \text { for } P(r, T)=\int_{0}^{T} 4,000 e^{-r t} d t$$ Dwijendra R. Numerade Educator ### Problem 30 Find the indicated value of the given function. $$F(0.07,10) \text { for } F(r, T)=\int_{0}^{T} 4,000 e^{r(T-t)} d t$$ Dwijendra R. Numerade Educator ### Problem 31 Find the indicated function$f$of a single variable. $$f(x)=G(x, 0) \text { for } G(x, y)=x^{2}+3 x y+y^{2}-7$$ Dwijendra R. Numerade Educator ### Problem 32 Find the indicated function$f$of a single variable. $$f(y)=H(0, y) \text { for } H(x, y)=x^{2}-5 x y-y^{2}+2$$ Dwijendra R. Numerade Educator ### Problem 33 Find the indicated function$f$of a single variable. $$f(y)=K(4, y) \text { for } K(x, y)=10 x y+3 x-2 y+8$$ Dwijendra R. Numerade Educator ### Problem 34 Find the indicated function$f$of a single variable. $$f(x)=L(x,-2) \text { for } L(x, y)=25-x+5 y-6 x y$$ Dwijendra R. Numerade Educator ### Problem 35 Find the indicated function$f$of a single variable. $$f(y)=M(y, y) \text { for } M(x, y)=x^{2} y-3 x y^{2}+5$$ Dwijendra R. Numerade Educator ### Problem 36 Find the indicated function$f$of a single variable. $$f(x)=N(x, 2 x) \text { for } N(x, y)=3 x y+x^{2}-y^{2}+1$$ Dwijendra R. Numerade Educator ### Problem 37 Let$F(x, y)=2 x+3 y-6$. Find all values of$y$such that$F(0, y)=0$. Dwijendra R. Numerade Educator ### Problem 38 Let$F(x, y)=5 x-4 y+12$. Find all values of$x$such that$F(x, 0)=0$. Dwijendra R. Numerade Educator ### Problem 39 Let$F(x, y)=2 x y+3 x-4 y-1$. Find all values of$x$such that$F(x, x)=0$. Dwijendra R. Numerade Educator ### Problem 40 Let$F(x, y)=x y+2 x^{2}+y^{2}-25 .$Find all values of$y$such that$F(y, y)=0$. Dwijendra R. Numerade Educator ### Problem 41 Let$F(x, y)=x^{2}+e^{x} y-y^{2} .$Find all values of$x$such that$F(x, 2)=0$. Dwijendra R. Numerade Educator ### Problem 42 Let$G(a, b, c)=a^{3}+b^{3}+c^{3}-(a b+a c+b c)-6$. Find all values of$b$such that$G(2, b, 1)=0$. Dwijendra R. Numerade Educator ### Problem 43 For the function$f(x, y)=x^{2}+2 y^{2},$find $$\frac{f(x+h, y)-f(x, y)}{h}$$ Dwijendra R. Numerade Educator ### Problem 44 For the function$f(x, y)=x^{2}+2 y^{2},$find $$\frac{f(x, y+k)-f(x, y)}{k}$$ Dwijendra R. Numerade Educator ### Problem 45 For the function$f(x, y)=2 x y^{2},$find $$\frac{f(x+h, y)-f(x, y)}{h}$$ Dwijendra R. Numerade Educator ### Problem 46 For the function$f(x, y)=2 x y^{2},$find $$\frac{f(x, y+k)-f(x, y)}{k}$$ Dwijendra R. Numerade Educator ### Problem 47 Find the coordinates of$E$and$F$in the figure for Matched Problem 6 on page$429 .$Dwijendra R. Numerade Educator ### Problem 48 Find the coordinates of$B$and$H$in the figure for Matched Problem 6 on page$429 .$Dwijendra R. Numerade Educator ### Problem 49 Use a graphing calculator as necessary to explore the graphs of the indicated cross sections. Let$f(x, y)=x^{2}$(A) Explain why the cross sections of the surface$z=f(x, y)$produced by cutting it with planes parallel to$y=0$are parabolas. (B) Describe the cross sections of the surface in the planes$x=0, x=1,$and$x=2$. (C) Describe the surface$z=f(x, y)$. Sunanda A. Numerade Educator ### Problem 50 Use a graphing calculator as necessary to explore the graphs of the indicated cross sections. Let$f(x, y)=\sqrt{4-y^{2}}$(A) Explain why the cross sections of the surface$z=f(x, y)$produced by cutting it with planes parallel to$x=0$are semicircles of radius 2 . (B) Describe the cross sections of the surface in the planes$y=0, y=2,$and$y=3$. (C) Describe the surface$z=f(x, y)$. Sunanda A. Numerade Educator ### Problem 51 Use a graphing calculator as necessary to explore the graphs of the indicated cross sections. Let$f(x, y)=\sqrt{36-x^{2}-y^{2}}$(A) Describe the cross sections of the surface$z=f(x, y)$produced by cutting it with the planes$y=1, y=2y=3, y=4,$and$y=5$. (B) Describe the cross sections of the surface in the planes$x=0, x=1, x=2, x=3, x=4,$and$x=5$. (C) Describe the surface$z=f(x, y)$. Sunanda A. Numerade Educator ### Problem 52 Use a graphing calculator as necessary to explore the graphs of the indicated cross sections. Let$f(x, y)=100+10 x+25 y-x^{2}-5 y^{2}$(A) Describe the cross sections of the surface$z=f(x, y)$produced by cutting it with the planes$y=0, y=1$,$y=2,$and$y=3$. (B) Describe the cross sections of the surface in the planes$x=0, x=1, x=2,$and$x=3$. (C) Describe the surface$z=f(x, y)$. Sunanda A. Numerade Educator ### Problem 53 Use a graphing calculator as necessary to explore the graphs of the indicated cross sections. Let$f(x, y)=e^{-\left(x^{2}+y^{2}\right)}$(A) Explain why$f(a, b)=f(c, d)$whenever$(a, b)$and$(c, d)$are points on the same circle centered at the origin in the$x y$plane. (B) Describe the cross sections of the surface$z=f(x, y)$produced by cutting it with the planes$x=0, y=0$, and$x=y$. (C) Describe the surface$z=f(x, y)$. Sunanda A. Numerade Educator ### Problem 54 Use a graphing calculator as necessary to explore the graphs of the indicated cross sections. Let$f(x, y)=4-\sqrt{x^{2}+y^{2}}$(A) Explain why$f(a, b)=f(c, d)$whenever$(a, b)$and$(c, d)$are points on the same circle with center at the origin in the$x y$plane. (B) Describe the cross sections of the surface$z=f(x, y)$produced by cutting it with the planes$x=0, y=0$, and$x=y$. (C) Describe the surface$z=f(x, y)$. Sunanda A. Numerade Educator ### Problem 55 A small manufacturing company produces two models of a surfboard: a standard model and a competition model. If the standard model is produced at a variable cost of$\$210$ each and the competition model at a variable cost of $\$ 300$each, and if the total fixed costs per month are$\$6,000,$ then the monthly cost function is given by
$$C(x, y)=6,000+210 x+300 y$$
where $x$ and $y$ are the numbers of standard and competition models produced per month, respectively. Find $C(20,10)$ $C(50,5),$ and $C(30,30)$.

Dwijendra R.

### Problem 56

A company spends $\$ x$thousand per week on online advertising and \$y thousand per week on TV advertising. Its weekly sales are found to be given by
$$S(x, y)=5 x^{2} y^{3}$$
Find $S(3,2)$ and $S(2,3)$.

Dwijendra R.

### Problem 57

Revenue function. A supermarket sells two brands of coffee: brand $A$ at $\$ p$per pound and brand$B$at$\$q$ per pound. The daily demand equations for brands $A$ and $B$ are, respectively,
$$\begin{array}{l} x=200-5 p+4 q \\ y=300+2 p-4 q \end{array}$$
(both in pounds). Find the daily revenue function $R(p, q)$. Evaluate $R(2,3)$ and $R(3,2)$.

Dwijendra R.

### Problem 58

A company manufactures 10 - and 3 -speed bicycles. The weekly demand and cost equations are
\begin{aligned} p &=230-9 x+y \\ q &=130+x-4 y \\ C(x, y) &=200+80 x+30 y \end{aligned}
where $\$ p$is the price of a 10 -speed bicycle,$\$q$ is the price of a 3-speed bicycle, $x$ is the weekly demand for 10 -speed bicycles, $y$ is the weekly demand for 3 -speed bicycles, and $C(x, y)$ is the cost function. Find the weekly revenue function $R(x, y)$ and the weekly profit function $P(x, y)$. Evaluate $R(10,15)$ and $P(10,15)$.

Dwijendra R.

### Problem 59

Productivity. The Cobb-Douglas production function for a petroleum company is given by
$$f(x, y)=20 x^{0.4} y^{0.6}$$
where $x$ is the utilization of labor and $y$ is the utilization of capital. If the company uses 1,250 units of labor and 1,700 units of capital, how many units of petroleum will be produced?

Dwijendra R.

### Problem 60

Productivity. The petroleum company in Problem 59 is taken over by another company that decides to double both the units of labor and the units of capital utilized in the production of petroleum. Use the Cobb-Douglas production function given in Problem 59 to find the amount of petroleum that will be produced by this increased utilization of labor and capital. What is the effect on productivity of doubling both the units of labor and the units of capital?

Dwijendra R.

### Problem 61

Future value. At the end of each year, $\$ 5,000$is invested into an IRA earning$3 \%$compounded annually. (A) How much will be in the account at the end of 30 years? Use the annuity formula $$F(P, i, n)=P \frac{(1+i)^{n}-1}{i}$$ where$P=$periodic payment$i=$rate per period$n=$number of payments (periods)$F=\mathrm{FV}=$future value (B) Use graphical approximation methods to determine the rate of interest that would produce$\$300,000$ in the account at the end of 30 years.

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### Problem 62

Package design. The packaging department in a company has been asked to design a rectangular box with no top and a partition down the middle (see the figure). Let $x, y,$ and $z$ be the dimensions of the box (in inches). Ignore the thickness of the material from which the box will be made.
(A) Find the total area of material $M(x, y, z)$ used in constructing one of these boxes, and evaluate $M(10,12,6)$.
(B) Suppose that the box will have a square base and a volume of 720 cubic inches. Use graphical approximation methods to determine the dimensions that require the least material.

Sunanda A.

### Problem 63

Marine biology. For a diver using scuba-diving gear, a marine biologist estimates the time (duration) of a dive according to the equation
$$T(V, x)=\frac{33 V}{x+33}$$
where
$T=$ time of dive in minutes
$V=$ volume of air, at sea level pressure, compressed into tanks
$x=$ depth of dive in feet
Find $T(70,47)$ and $T(60,27)$.

Dwijendra R.

### Problem 64

Blood flow. Poiseuille's law states that the resistance $R$ for blood flowing in a blood vessel varies directly as the length $L$ of the vessel and inversely as the fourth power of its radius $r$. Stated as an equation,
$$R(L, r)=k \frac{L}{r^{4}} \quad k \text { a constant }$$
Find $R(8,1)$ and $R(4,0.2)$.

Dwijendra R.

### Problem 65

Physical anthropology. Anthropologists use an index called the cephalic index. The cephalic index $C$ varies directly as the width $W$ of the head and inversely as the length $L$ of the head (both viewed from the top). In terms of an equation, $C(W, L)=100 \frac{W}{L}$
where
$$\begin{array}{c} W=\text { width in inches } \\ L=\text { length in inches } \\ \text { Find } C(6,8) \text { and } C(8.1,9) . \end{array}$$

Dwijendra R.

### Problem 66

Under ideal conditions, if a person driving a car slams on the brakes and skids to a stop, the length of the skid marks (in feet) is given by the formula
$$L(w, v)=k w v^{2}$$
where
\begin{aligned} k &=\text { constant } \\ w &=\text { weight of car in pounds } \\ v &=\text { speed of car in miles per hour } \end{aligned}
For $k=0.0000133,$ find $L(2,000,40)$ and $L(3,000,60)$.

Dwijendra R.

### Problem 67

Psychology. The intelligence quotient (IQ) is defined to be the ratio of mental age (MA), as determined by certain tests, to chronological age (CA), multiplied by $100 .$ Stated as an equation,
$$Q(M, C)=\frac{M}{C} \cdot 100$$
where
$$Q=\mathrm{IQ} \quad M=\mathrm{MA} \quad C=\mathrm{CA}$$
Find $Q(12,10)$ and $Q(10,12)$.

Dwijendra R.