# EXAMPLE 5 Find the area of the largest rectangle that can be

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```					312    I     CHAPTER 4 APPLICATIONS OF DIFFERENTIATION

EXAMPLE 5 Find the area of the largest rectangle that can be inscribed in a semicircle
Resources / Module 5                               SOLUTION 1 Let’s take the semicircle to be the upper half of the circle x 2 y2 r2
/ Max and Min
/ Start of Max and Min
with center the origin. Then the word inscribed means that the rectangle has two
vertices on the semicircle and two vertices on the x-axis as shown in Figure 9.
y                                          Let x, y be the vertex that lies in the ﬁrst quadrant. Then the rectangle has sides
of lengths 2x and y, so its area is
A 2xy

(x, y)          To eliminate y we use the fact that x, y lies on the circle x 2                                              y2        r 2 and so
2x                  y                    y sr 2 x 2. Thus
A 2xsr 2 x 2
_r               0                       r       x
The domain of this function is 0                      x       r. Its derivative is
FIGURE 9
2x 2                  2 r 2 2x 2
A         2sr 2         x2
sr   2       x    2
sr 2 x 2
which is 0 when 2x 2 r 2, that is, x r s2 (since x 0). This value of x gives a
maximum value of A since A 0        0 and A r  0. Therefore, the area of the largest
inscribed rectangle is
r                   r                        r2
A                      2                r2                      r2
s2                  s2                        2

SOLUTION 2 A simpler solution is possible if we think of using an angle as a variable.
Let   be the angle shown in Figure 10. Then the area of the rectangle is
r                                               A               2r cos         r sin              r 2 2 sin             cos                r 2 sin 2
r Ã ¨

¨                                  We know that sin 2 has a maximum value of 1 and it occurs when 2                2. So
A has a maximum value of r 2 and it occurs when               4.
r Ł ¨
Notice that this trigonometric solution doesn’t involve differentiation. In fact, we
FIGURE 10                                                 didn’t need to use calculus at all.

4.6       Exercises           G      G       G       G     G   G     G    G   G       G      G     G      G        G    G       G            G   G      G        G     G    G      G   G     G   G

1. Consider the following problem: Find two numbers whose                                                   always 23. On the basis of the evidence in your table,
sum is 23 and whose product is a maximum.                                                                estimate the answer to the problem.
(a) Make a table of values, like the following one, so that                                          (b) Use calculus to solve the problem and compare with
the sum of the numbers in the ﬁrst two columns is                                                    your answer to part (a).

First number              Second number                  Product                       2. Find two numbers whose difference is 100 and whose prod-
uct is a minimum.
1                            22                      22
2                            21                      42                          3. Find two positive numbers whose product is 100 and whose
3                            20                      60                                sum is a minimum.
.                             .                       .
.                             .                       .                          4. Find a positive number such that the sum of the number and
.                             .                       .
its reciprocal is as small as possible.
SECTION 4.6 OPTIMIZATION PROBLEMS            N     313

5. Find the dimensions of a rectangle with perimeter 100 m          12. A rectangular storage container with an open top is to have
whose area is as large as possible.                                   a volume of 10 m3. The length of its base is twice the width.
Material for the base costs \$10 per square meter. Material
6. Find the dimensions of a rectangle with area 1000 m2 whose
for the sides costs \$6 per square meter. Find the cost of
perimeter is as small as possible.
materials for the cheapest such container.
7. Consider the following problem: A farmer with 750 ft of
fencing wants to enclose a rectangular area and then divide       13. Find the point on the line y        4x    7 that is closest to the
it into four pens with fencing parallel to one side of the            origin.
rectangle. What is the largest possible total area of the four    14. Find the point on the parabola x         y2   0 that is closest to
pens?                                                                 the point 0,     3.
(a) Draw several diagrams illustrating the situation, some
with shallow, wide pens and some with deep, narrow           15. Find the dimensions of the rectangle of largest area that can
pens. Find the total areas of these conﬁgurations. Does          be inscribed in an equilateral triangle of side L if one side
it appear that there is a maximum area? If so, estimate          of the rectangle lies on the base of the triangle.
it.                                                          16. Find the dimensions of the rectangle of largest area that has
(b) Draw a diagram illustrating the general situation. Intro-         its base on the x-axis and its other two vertices above the
duce notation and label the diagram with your symbols.           x-axis and lying on the parabola y 8 x 2.
(c) Write an expression for the total area.
(d) Use the given information to write an equation that           17. A right circular cylinder is inscribed in a sphere of radius r.
relates the variables.                                           Find the largest possible surface area of such a cylinder.
(e) Use part (d) to write the total area as a function of one     18. Find the area of the largest rectangle that can be inscribed in
variable.                                                        the ellipse x 2 a 2   y2 b2    1.
(f) Finish solving the problem and compare the answer
with your estimate in part (a).                              19. A Norman window has the shape of a rectangle surmounted
by a semicircle. (Thus the diameter of the semicircle is
8. Consider the following problem: A box with an open top is            equal to the width of the rectangle. See Exercise 48 on
to be constructed from a square piece of cardboard, 3 ft              page 24.) If the perimeter of the window is 30 ft, ﬁnd the
wide, by cutting out a square from each of the four corners           dimensions of the window so that the greatest possible
and bending up the sides. Find the largest volume that such           amount of light is admitted.
a box can have.
(a) Draw several diagrams to illustrate the situation, some       20. A right circular cylinder is inscribed in a cone with height h
short boxes with large bases and some tall boxes with             and base radius r. Find the largest possible volume of such
small bases. Find the volumes of several such boxes.              a cylinder.
Does it appear that there is a maximum volume? If so,         21. A piece of wire 10 m long is cut into two pieces. One piece
estimate it.                                                      is bent into a square and the other is bent into an equilateral
(b) Draw a diagram illustrating the general situation. Intro-         triangle. How should the wire be cut so that the total area
duce notation and label the diagram with your symbols.            enclosed is (a) a maximum? (b) A minimum?
(c) Write an expression for the volume.
(d) Use the given information to write an equation that           22. A fence 8 ft tall runs parallel to a tall building at a distance
relates the variables.                                            of 4 ft from the building. What is the length of the shortest
(e) Use part (d) to write the volume as a function of one             ladder that will reach from the ground over the fence to the
variable.                                                         wall of the building?
(f) Finish solving the problem and compare the answer             23. A conical drinking cup is made from a circular piece of
with your estimate in part (a).                                   paper of radius R by cutting out a sector and joining the
9. If 1200 cm2 of material is available to make a box with a            edges CA and CB. Find the maximum capacity of such a
square base and an open top, ﬁnd the largest possible                 cup.
volume of the box.
A               B
10. A box with a square base and open top must have a volume
of 32,000 cm3. Find the dimensions of the box that mini-                                                  R
mize the amount of material used.
11. (a) Show that of all the rectangles with a given area, the one                                      C
with smallest perimeter is a square.
(b) Show that of all the rectangles with a given perimeter,
the one with greatest area is a square.
314     I    CHAPTER 4 APPLICATIONS OF DIFFERENTIATION

24. For a ﬁsh swimming at a speed v relative to the water, the              26. A boat leaves a dock at 2:00 P.M. and travels due south at
energy expenditure per unit time is proportional to v 3. It is              a speed of 20 km h. Another boat has been heading due
believed that migrating ﬁsh try to minimize the total energy                east at 15 km h and reaches the same dock at 3:00 P.M. At
required to swim a ﬁxed distance. If the ﬁsh are swimming                   what time were the two boats closest together?
against a current u u v , then the time required to swim a
distance L is L v u and the total energy E required to                  27. The illumination of an object by a light source is directly
swim the distance is given by                                               proportional to the strength of the source and inversely
proportional to the square of the distance from the source.
L                               If two light sources, one three times as strong as the other,
Ev         av 3
v       u                           are placed 10 ft apart, where should an object be placed on
where a is the proportionality constant.                                    the line between the sources so as to receive the least
illumination?
(a) Determine the value of v that minimizes E.
(b) Sketch the graph of E.                                              28. A woman at a point A on the shore of a circular lake with
Note: This result has been veriﬁed experimentally;                          radius 2 mi wants to arrive at the point C diametrically
migrating ﬁsh swim against a current at a speed 50%                         opposite A on the other side of the lake in the shortest pos-
greater than the current speed.                                             sible time. She can walk at the rate of 4 mi h and row a
boat at 2 mi h. How should she proceed?
25. In a beehive, each cell is a regular hexagonal prism, open at
one end with a trihedral angle at the other end. It is believed                                                      B
that bees form their cells in such a way as to minimize the
surface area for a given volume, thus using the least amount
of wax in cell construction. Examination of these cells has
shown that the measure of the apex angle is amazingly                                             ¨
A                             C
consistent. Based on the geometry of the cell, it can be                                           2             2
shown that the surface area S is given by

S    6sh
3 2
2s cot           (3s 2s3 2) csc
where s, the length of the sides of the hexagon, and h, the
height, are constants.                                                  29. Find an equation of the line through the point 3, 5 that
(a) Calculate dS d .                                                        cuts off the least area from the ﬁrst quadrant.
(b) What angle should the bees prefer?                            CAS   30. The frame for a kite is to be made from six pieces of wood.
(c) Determine the minimum surface area of the cell (in                      The four exterior pieces have been cut with the lengths indi-
terms of s and h).                                                      cated in the ﬁgure. To maximize the area of the kite, how
Note: Actual measurements of the angle in beehives have                     long should the diagonal pieces be?
been made, and the measures of these angles seldom differ
from the calculated value by more than 2 .
a            a

rear                    trihedral
of cell                    angle ¨

b            b

h

b                                      ; 31. A point P needs to be located somewhere on the line AD so
front                       that the total length L of cables linking P to the points A, B,
s               of cell                     and C is minimized (see the ﬁgure). Express L as a function
SECTION 4.6 OPTIMIZATION PROBLEMS        N      315

of x    AP and use the graphs of L and dL dx to estimate           34. Two vertical poles PQ and ST are secured by a rope PRS
the minimum value.                                                     going from the top of the ﬁrst pole to a point R on the
ground between the poles and then to the top of the second
A                                       pole as in the ﬁgure. Show that the shortest length of such a
rope occurs when 1       2.

P
5m                           P
S

2m           3m
B         D                      C

32. The graph shows the fuel consumption c of a car (measured
in gallons per hour) as a function of the speed v of the car.                                      ¨¡           ¨™
At very low speeds the engine runs inefﬁciently, so initially
Q                        R        T
c decreases as the speed increases. But at high speeds the
fuel consumption increases. You can see that c v is mini-
mized for this car when v 30 mi h. However, for fuel
35. The upper left-hand corner of a piece of paper 8 in. wide by
efﬁciency, what must be minimized is not the consumption
in gallons per hour but rather the fuel consumption in                 12 in. long is folded over to the right-hand edge as in the
gallons per mile. Let’s call this consumption G. Using the             ﬁgure. How would you fold it so as to minimize the length
graph, estimate the speed at which G has its minimum                   of the fold? In other words, how would you choose x to
value.                                                                 minimize y?

c
x

y

0                                        √                             12
20        40        60

33. Let v1 be the velocity of light in air and v2 the velocity of
light in water. According to Fermat’s Principle, a ray of
light will travel from a point A in the air to a point B in the
water by a path ACB that minimizes the time taken. Show                                                 8
that
sin    1    v1                             36. A steel pipe is being carried down a hallway 9 ft wide. At
sin    2    v2                                 the end of the hall there is a right-angled turn into a
narrower hallway 6 ft wide. What is the length of the
where 1 (the angle of incidence) and 2 (the angle of
longest pipe that can be carried horizontally around the
refraction) are as shown. This equation is known as Snell’s
corner?
Law.

A
¨¡                                                                                                   6

C
¨

¨™
B                         9
316     I    CHAPTER 4 APPLICATIONS OF DIFFERENTIATION

37. Find the maximum area of a rectangle that can be circum-        41. Ornithologists have determined that some species of birds
scribed about a given rectangle with length L and width W .         tend to avoid ﬂights over large bodies of water during day-
light hours. It is believed that more energy is required to ﬂy
over water than land because air generally rises over land
and falls over water during the day. A bird with these
¨
tendencies is released from an island that is 5 km from the
L
nearest point B on a straight shoreline, ﬂies to a point C on
W                                             the shoreline, and then ﬂies along the shoreline to its nest-
ing area D. Assume that the bird instinctively chooses a
path that will minimize its energy expenditure. Points B and
D are 13 km apart.
(a) In general, if it takes 1.4 times as much energy to ﬂy
over water as land, to what point C should the bird ﬂy in
38. A rain gutter is to be constructed from a metal sheet of                order to minimize the total energy expended in returning
width 30 cm by bending up one-third of the sheet on each                to its nesting area?
side through an angle . How should be chosen so that the            (b) Let W and L denote the energy (in joules) per kilometer
gutter will carry the maximum amount of water?                          ﬂown over water and land, respectively. What would a
large value of the ratio W L mean in terms of the bird’s
ﬂight? What would a small value mean? Determine the
ratio W L corresponding to the minimum expenditure of
¨                             ¨                           energy.
(c) What should the value of W L be in order for the bird to
10 cm          10 cm         10 cm                           ﬂy directly to its nesting area D? What should the value
of W L be for the bird to ﬂy to B and then along the
39. Where should the point P be chosen on the line segment AB               shore to D?
so as to maximize the angle ?                                       (d) If the ornithologists observe that birds of a certain
species reach the shore at a point 4 km from B, how
many times more energy does it take a bird to ﬂy over
water than land?

5                                                               island

2
¨                                          5 km
A                     B                                                       C              D
P                                             B
13 km              nest
3

40. A painting in an art gallery has height h and is hung so that
its lower edge is a distance d above the eye of an observer
(as in the ﬁgure). How far from the wall should the observer    42. The blood vascular system consists of blood vessels (arter-
stand to get the best view? (In other words, where should           ies, arterioles, capillaries, and veins) that convey blood from
the observer stand so as to maximize the angle subtended            the heart to the organs and back to the heart. This system
at his eye by the painting?)                                        should work so as to minimize the energy expended by the
heart in pumping the blood. In particular, this energy is
reduced when the resistance of the blood is lowered. One of
h                   Poiseuille’s Laws gives the resistance R of the blood as
¨                         d                                                       L
R       C
r4
SECTION 4.6 OPTIMIZATION PROBLEMS               N   317

where L is the length of the blood vessel, r is the radius,             mitted signals are recorded at a point Q, which is a distance
and C is a positive constant determined by the viscosity of             D from P. The ﬁrst signal to arrive at Q travels along the
the blood. (Poiseuille established this law experimentally              surface and takes T1 seconds. The next signal travels from
but it also follows from Equation 6.6.2.) The ﬁgure shows a             P to a point R, from R to S in the lower layer, and then to
main blood vessel with radius r1 branching at an angle                  Q, taking T2 seconds. The third signal is reﬂected off the
into a smaller vessel with radius r2.                                   lower layer at the midpoint O of RS and takes T3 seconds
to reach Q.
C              (a) Express T1, T2, and T3 in terms of D, h, c1, c2, and .
(b) Show that T2 is a minimum when sin           c1 c2.
(c) Suppose that D 1 km, T1 0.26 s, T2 0.32 s,
r™                                 T3 0.34 s. Find c1, c2, and h.
vascular                                                  b
branching                                                                          P                      D                    Q
A             r¡            ¨                                                                            Speed of sound=c¡
B                                                                 h       ¨                                   ¨

a
R          O            S
Speed of sound=c™
(a) Use Poiseuille’s Law to show that the total resistance of
the blood along the path ABC is
Note: Geophysicists use this technique when studying the
a       b cot        b csc                          structure of the earth’s crust, whether searching for oil or
R    C                                                       examining fault lines.
r41             r4
2

; 44. Two light sources of identical strength are placed 10 m
where a and b are the distances shown in the ﬁgure.                 apart. An object is to be placed at a point P on a line par-
(b) Prove that this resistance is minimized when                        allel to the line joining the light sources and at a distance of
4                                  d meters from it (see the ﬁgure). We want to locate P on
r2
cos          4
so that the intensity of illumination is minimized. We need
r1                                  to use the fact that the intensity of illumination for a single
source is directly proportional to the strength of the source
(c) Find the optimal branching angle (correct to the nearest
and inversely proportional to the square of the distance from
degree) when the radius of the smaller blood vessel is
the source.
two-thirds the radius of the larger vessel.
(a) Find an expression for the intensity I x at the point P.
(b) If d 5 m, use graphs of I x and I x to show that the
intensity is minimized when x 5 m, that is, when P is
at the midpoint of .
(c) If d 10 m, show that the intensity (perhaps surpris-
ingly) is not minimized at the midpoint.
(d) Somewhere between d 5 m and d 10 m there is a
transitional value of d at which the point of minimal
illumination abruptly changes. Estimate this value of d
by graphical methods. Then ﬁnd the exact value of d.

x
P
43. The speeds of sound c1 in an upper layer and c2 in a lower                                     d
layer of rock and the thickness h of the upper layer can be
determined by seismic exploration if the speed of sound in
the lower layer is greater than the speed in the upper layer.                                             10 m
A dynamite charge is detonated at a point P and the trans-

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