Calculus: Early Transcendentals 9th Edition

Published by Cengage Learning
ISBN 10: 1337613924
ISBN 13: 978-1-33761-392-7

Chapter 7 - Section 7.8 - Improper Integrals - 7.8 Exercises - Page 550: 59

Answer

Divergent

Work Step by Step

Define $f(x)=\frac{1}{x-\ln x}$ and $g(x)=\frac{1}{x}$. For $x\geq 2$, we have $\Leftrightarrow \ln x>0$ $\Leftrightarrow -\ln x< 0$ $\Leftrightarrow x-\ln x\frac{1}{x}$ $\Leftrightarrow f(x)>g(x)$ Find the convergence for $\int_2^\infty g(x) dx$: $\int_2^\infty \frac{1}{x} dx=\lim\limits_{t \to \infty}\int_2^t\frac{1}{x} dx=\lim\limits_{t \to \infty}[\ln x]_2^t=\lim\limits_{t \to \infty}[\ln t-\ln 2]=\infty$ Since $f(x)>g(x)$ for $x\geq 2$ and $\int_2^\infty g(x)$ is divergent, using the Comparison Theorem it concludes that $\int_2^\infty \frac{1}{x-\ln x} dx$ is divergent.
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