James Stewart-Calculus_ Early Transcendentals-Cengage Learning (2015)

250 Chapter 3 Differentiation Rules
21. The altitude of a triangle is increasing at a rate of 1 cmymin
while the area of the triangle is increasing at a rate of
2 cm 2 ymin. At what rate is the base of the triangle changing
when the altitude is 10 cm and the area is 100 cm 2 ?
22. A boat is pulled into a dock by a rope attached to the bow of
the boat and passing through a pulley on the dock that is 1 m
higher than the bow of the boat. If the rope is pulled in at a
rate of 1 mys, how fast is the boat approaching the dock
when it is 8 m from the dock?
the shape of a cone whose base diameter and height are
always equal. How fast is the height of the pile increasing
when the pile is 10 ft high?
23. At noon, ship A is 100 km west of ship B. Ship A is sailing
south at 35 kmyh and ship B is sailing north at 25 kmyh.
How fast is the distance between the ships changing at
4:00 pm?
24. A particle moves along the curve y − 2 sinsxy2d. As the
particle passes through the point ( 1 3 , 1), its x-coordinate
increases at a rate of s10 cmys. How fast is the distance
from the particle to the origin changing at this instant?
25. Water is leaking out of an inverted conical tank at a rate of
10,000 cm 3 ymin at the same time that water is being pumped
into the tank at a constant rate. The tank has height 6 m and
the diameter at the top is 4 m. If the water level is rising at a
rate of 20 cmymin when the height of the water is 2 m, find
the rate at which water is being pumped into the tank.
26. A trough is 10 ft long and its ends have the shape of isosceles
triangles that are 3 ft across at the top and have a height
of 1 ft. If the trough is being filled with water at a rate of
12 ft 3 ymin, how fast is the water level rising when the water
is 6 inches deep?
27. A water trough is 10 m long and a cross-section has the
shape of an isosceles trapezoid that is 30 cm wide at the
bottom, 80 cm wide at the top, and has height 50 cm. If the
trough is being filled with water at the rate of 0.2 m 3 ymin,
how fast is the water level rising when the water is 30 cm
deep?
28. A swimming pool is 20 ft wide, 40 ft long, 3 ft deep at the
shallow end, and 9 ft deep at its deepest point. A crosssection
is shown in the figure. If the pool is being filled at a
rate of 0.8 ft 3 ymin, how fast is the water level rising when
the depth at the deepest point is 5 ft?
6
12 16 6
29. Gravel is being dumped from a conveyor belt at a rate of
30 ft 3 ymin, and its coarseness is such that it forms a pile in
3
6
;
30. A kite 100 ft above the ground moves horizontally at a speed
of 8 ftys. At what rate is the angle between the string and the
horizontal decreasing when 200 ft of string has been let out?
31. The sides of an equilateral triangle are increasing at a rate of
10 cmymin. At what rate is the area of the triangle increasing
when the sides are 30 cm long?
32. How fast is the angle between the ladder and the ground
changing in Example 2 when the bottom of the ladder is 6 ft
from the wall?
33. The top of a ladder slides down a vertical wall at a rate of
0.15 mys. At the moment when the bottom of the ladder is
3 m from the wall, it slides away from the wall at a rate of
0.2 mys. How long is the ladder?
34. According to the model we used to solve Example 2, what
happens as the top of the ladder approaches the ground? Is
the model appropriate for small values of y?
35. If the minute hand of a clock has length r (in centimeters),
find the rate at which it sweeps out area as a function of r.
36. A faucet is filling a hemispherical basin of diameter 60 cm
with water at a rate of 2 Lymin. Find the rate at which the
water is rising in the basin when it is half full. [Use the
following facts: 1 L is 1000 cm 3 . The volume of the portion
of a sphere with radius r from the bottom to a height h is
V − (rh 2 2 1 3 h 3 ), as we will show in Chapter 6.]
37. Boyle’s Law states that when a sample of gas is compressed
at a constant temperature, the pressure P and volume V
satisfy the equation PV − C, where C is a constant. Suppose
that at a certain instant the volume is 600 cm 3 , the pressure
is 150 kPa, and the pressure is increasing at a rate of
20 kPaymin. At what rate is the volume decreasing at this
instant?
38. When air expands adiabatically (without gaining or losing
heat), its pressure P and volume V are related by the
equation PV 1.4 − C, where C is a constant. Suppose that at
a certain instant the volume is 400 cm 3 and the pressure is
80 kPa and is decreasing at a rate of 10 kPaymin. At what
rate is the volume increasing at this instant?
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