Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 4 - Section 4.3 - How Derivatives Affect the Shape of a Graph - 4.3 Exercises - Page 302: 55

Answer

(a) $x=0$ and $x=e$ are vertical asymptotes. (b) $f$ is decreasing on the interval $(0, e)$ (c) There is no local maximum or local minimum. (d) The graph is concave down on this interval: $(1, e)$ The graph is concave up on this interval: $(0, 1)$ The point of inflection is $(1, 0)$ (e) We can see a sketch of the graph below.

Work Step by Step

(a) $f(x) = ln~(1-ln~x)$ $f(x)$ is defined for values of $x$ such that $0 \lt x$ and $(1-ln~x) \gt 0$ $f(x)$ is defined for values of $x$ such that $0 \lt x$ and $x \lt e$ $x=0$ and $x=e$ are vertical asymptotes. $\lim\limits_{x \to \infty}f(x) = -\infty$ There is no horizontal asymptote. (b) We can find the points where $f'(x) = 0$: $f'(x) = \frac{1}{1-ln~x}\cdot (-\frac{1}{x})$ $f'(x) = -\frac{1}{x~(1-ln~x)} = 0$ There are no values of $x$ such that $f'(x) = 0$ When $0 \lt x$ then $f'(x) \lt 0$ $f$ is decreasing on the interval $(0, e)$ (the domain) $f$ is not increasing on any interval. (c) There are no values of $x$ such that $f'(x) = 0$ There is no local maximum or local minimum. (d) We can find the points where $f''(x) = 0$: $f''(x) = -\frac{-(1-ln~x)-(x)(-\frac{1}{x})}{x^2~(1-ln~x)^2}$ $f''(x) = \frac{1-ln~x-1}{x^2~(1-ln~x)^2}$ $f''(x) = -\frac{ln~x}{x^2~(1-ln~x)^2} = 0$ $ln~x = 0$ $x = 1$ The graph is concave down when $f''(x) \lt 0$ The graph is concave down on this interval: $(1, e)$ The graph is concave up when $f''(x) \gt 0$ The graph is concave up on this interval: $(0, 1)$ $f(1) = ln~(1-ln~1) = ln(1) = 0$ The point of inflection is $(1, 0)$ (e) We can see a sketch of the graph below.
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