5128_Ch03_pp098-184

40.
2x
, all reals
x
2
1
39. f (x) ln (2 cos x)
40. f (x) ln (x 2 1)
41. f (x) log 2 (3x 1)
42. f (x) log 10 x
1
42.
1
, x 1
2(x 1)ln 10
sin x
, all reals
2 cos x
3
(3x
, x 13
1) ln 2
Group Activity In Exercises 43–48, use the technique of logarithmic
differentiation to find dydx.
43. y sin x x , 0 x p2 (sin x) x [x cot x ln (sin x)]
44. y x tan x , x 0 x tan x tan x
(ln x)(sec 2 x)
x
45. y 5
x 3
2 x 4
x 2 5 3 1
46. y x x
2 1
x 1 2 3 x x 2 1
(x 1
) 2/ 3 1 x x 2
x 2
1 3(x 1)
47. y x ln x 2xln x (ln x)
48. y x (1/ln x) 0, x 0
x
49. Find an equation for a line that is tangent to the graph of y e x
and goes through the origin. y ex
50. Find an equation for a line that is normal to the graph of y xe x
and goes through the origin. y x
51. Spread of a Rumor The spread of a rumor in a certain school
is modeled by the equation
300
P(t) ,
1 24t
where P(t) is the total number of students who have heard the
rumor t days after the rumor first started to spread.
(a) Estimate the initial number of students who first heard the
rumor. 18
(b) How fast is the rumor spreading after 4 days?
(c) When will the rumor spread at its maximum rate? What is
that rate? After 4 days; 52 students per day
52. Spread of Flu The spread of flu in a certain school is modeled
by the equation
200
P(t) ,
1 e5t
where P(t) is the total number of students infected t days after
the flu first started to spread.
(a) Estimate the initial number of students infected with this flu. 1
(b) How fast is the flu spreading after 4 days? 39 students per day
(c) When will the flu spread at its maximum rate? What is that
rate? After 5 days; 50 students per day
53. Radioactive Decay The amount A (in grams) of radioactive
plutonium remaining in a 20-gram sample after t days is given
by the formula
A 20 • 12 t140 .
At what rate is the plutonium decaying when t 2 days?
Answer in appropriate units. rate 0.098 grams/day
54. For any positive constant k, the derivative of ln kx is 1x.
Prove this fact
(a) by using the Chain Rule. See page 180.
(b) by using a property of logarithms and differentiating.
45. 3)
4 (x
(x (2x
2
1)
5)
3
4 2x
6
1/5 5(x
3) 5(x
2
1) 5(2x 5)
Section 3.9 Derivatives of Exponential and Logarithmic Functions 179
See page 180.
52 students per day
55. Let f x 2 x .
(a) Find f 0. ln 2
(b) Use the definition of the derivative to write f 0 as a limit.
(c) Deduce the exact value of
f (0) lim 2h 1
h→0 h
1
lim
h→0 2h .
h
ln 2
(d) What is the exact value of
1
lim
h→0 7h ? ln 7
h
56. Writing to Learn The graph of y ln x looks as though it
might be approaching a horizontal asymptote. Write an argument
based on the graph of y e x to explain why it does not.
[–3, 6] by [–3, 3]
Standardized Test Questions
You should solve the following problems without using a
graphing calculator.
57. True or False The derivative of y 2 x is 2 x . Justify your
answer. False. It is (ln 2)2 x .
58. True or False The derivative of y e 2x is 2 (ln 2) e 2x . Justify
your answer. False. It is 2e 2x .
59. Multiple Choice If a flu is spreading at the rate of
150
P(t) ,
1 e4t
which of the following is the initial number of persons infected?
B
(A) 1 (B) 3 (C) 7 (D) 8 (E) 75
60. Multiple Choice Which of the following is the domain of
f (x) if f (x) log 2 (x 3)? D
(A) x 3 (B) x 3 (C) x 3 (D) x 3
(E) x 3
61. Multiple Choice Which of the following gives dydx if
y log 10 (2x 3)? A
2
2
1
(A) (B) (C)
(2x 3)ln 10 2x 3 (2x 3)ln10
1
1
(D) (E)
2x 3
2 x
62. Multiple Choice Which of the following gives the slope of
the tangent line to the graph of y 2 1x at x 2? E
(A) 1 2 (B) 1 2 (C) 2 (D) 2 (E) ln 2
2