Calculus: Early Transcendentals 8th Edition

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

Chapter 7 - Section 7.8 - Improper Integrals - 7.8 Exercises - Page 534: 24

Answer

$1$ (converges)

Work Step by Step

$\displaystyle \int\frac{1}{x(\ln x)^{2}}dx=\left[\begin{array}{l} u=\ln x\\ du=\frac{1}{x}dx \end{array}\right]=\int u^{-2}du=\frac{u^{-1}}{-1}+C$ $=-\displaystyle \frac{1}{\ln x}+C$ The integral is a type I improper integral, so $\displaystyle \int_{e}^{\infty}\frac{1}{x(\ln x)^{2}}dx=\lim_{t\rightarrow\infty}\int_{e}^{t}\frac{1}{x(\ln x)^{2}}dx$ $=\displaystyle \lim_{t\rightarrow\infty}\left[-\frac{1}{\ln x}\right]_{e}^{t}$ $=0-(-1)$ $=1$ (converges)
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