5128_Ch07_pp378-433

Chapter 7 Review Exercises 433
40. Yellow Perch A researcher measures the lengths of 3-year-old
yellow perch in a fish hatchery and finds that they have a mean
length of 17.2 cm with a standard deviation of 3.4 cm. What
proportion of 3-year-old yellow perch raised under similar
conditions can be expected to reach a length of 20 cm or more?
0.2051 (20.5%)
41. Group Activity Using as large a sample of classmates as
possible, measure the span of each person’s fully stretched hand,
from the tip of the pinky finger to the tip of the thumb. Based on
the mean and standard deviation of your sample, what percentage
of students your age would have a finger span of more than 10
inches? Answers will vary.
42. The 68-95-99.7 Rule (a) Verify that for every normal pdf, the
proportion of the population lying within one standard deviation
of the mean is close to 68%. Hint: Since it is the same for every
pdf, you can simplify the function by assuming that m 0 and
s 1. Then integrate from 1 to 1. (a) 0.6827 (68.27%)
(b) 0.9545 (95.45%)
(b) Verify the two remaining parts of the rule.
43. Writing to Learn Explain why the area under the graph of a
probability density function has to equal 1. The probability that
the variable has some value in the range of all possible values is 1.
In Exercises 44–48, use the cylindrical shell method to find the
volume of the solid generated by revolving the region bounded by
the curves about the y-axis.
44. y 2x, y x2, x 1 p
45. y 1x, y 0, x 12, x 2 3p
46. y sin x, y 0, 0 x p 2p 2
47. y x 3, y x 2 3x 16p/3
48. the bell-shaped region in Exercise 18 9.7717
49. Bundt Cake A bundt cake (see Exploration 1, Section 7.3) has
a hole of radius 2 inches and an outer radius of 6 inches at the
base. It is 5 inches high, and each cross-sectional slice is
parabolic.
(a) Model a typical slice by finding the equation of the parabola
with y-intercept 5 and x-intercepts 2. y 5 5 4 x 2
(b) Revolve the parabolic region about an appropriate line
to generate the bundt cake and find its volume. 335.1032 in 3
50. Finding a Function Find a function f that has a continuous
derivative on 0, ∞ and that has both of the following
properties.
i. The graph of f goes through the point 1, 1.
ii. The length L of the curve from 1, 1 to any point
x, f x is given by the formula L ln x f x 1.
In Exercises 51 and 52, find the area of the surface generated by
revolving the curve about the indicated axis.
51. y tan x, 0 x p4; x-axis 3.84
52. xy 1, 1 y 2; y-axis 5.02
AP* Examination Preparation
You may use a graphing calculator to solve the following
problems.
53. Let R be the region in the first quadrant enclosed by the y-axis
and the graphs of y 2 sin x and y sec x.
(a) Find the area of R.
(b) Find the volume of the solid generated when R is revolved
about the x-axis.
(c) Find the volume of the solid whose base is R and whose cross
sections cut by planes perpendicular to the x-axis are squares.
54. The temperature outside a house during a 24-hour period is given by
pt
F(t) 80 10 cos
1 2
,0 t 24,
where F(t) is measured in degrees Fahrenheit and t is measured
in hours.
(a) Find the average temperature, to the nearest degree Fahrenheit,
between t 6 and t 14.
(b) An air conditioner cooled the house whenever the outside
temperature was at or above 78 degrees Fahrenheit. For what
values of t was the air conditioner cooling the house?
(c) The cost of cooling the house accumulates at the rate of
$0.05 per hour for each degree the outside temperature exceeds
78 degrees Fahrenheit. What was the total cost, to the nearest
cent, to cool the house for this 24-hour period?
55. The rate at which people enter an amusement park on a given
day is modeled by the function E defined by
15600
E(t) t 2 .
24t
160
The rate at which people leave the same amusement park on the
same day is modeled by the function L defined by
9890
L(t) t 2 .
38t
370
Both E(t) and L(t) are measured in people per hour, and time t is
measured in hours after midnight. These functions are valid for
9 t 23, which are the hours that the park is open. At time
t 9, there are no people in the park.
(a) How many people have entered the park by 5:00 P.M.
(t 17)? Round your answer to the nearest whole number.
(b) The price of admission to the park is $15 until 5:00 P.M.
(t 17). After 5:00 P.M., the price of admission to the park is $11.
How many dollars are collected from admissions to the park on
the given day? Round your answer to the nearest whole number.
(c) Let H(t) t (E(x) L(x))dx for 9 t 23. The value of
9
H(17) to the nearest whole number is 3725. Find the value of
H(17) and explain the meaning of H(17) and H(17) in the context
of the park.
(d) At what time t, for 9 t 23, does the model predict that
the number of people in the park is a maximum?
50. f (x) x2 2 ln x 3
4