Calculus 10th Edition

Published by Brooks Cole
ISBN 10: 1-28505-709-0
ISBN 13: 978-1-28505-709-5

Chapter 8 - Integration Techniques, L'Hopital's Rule, and Improper Integrals - 8.8 Exercises - Page 575: 26

Answer

$${\text{The improper integral diverges}}$$

Work Step by Step

$$\eqalign{ & \int_1^\infty {\frac{{\ln x}}{x}dx} \cr & {\text{By the Definition of Improper Integrals with Infinite }} \cr & {\text{Integration Limits}} \cr & \int_a^\infty {f\left( x \right)dx = \mathop {\lim }\limits_{b \to \infty } } \int_a^b {f\left( x \right)} dx,{\text{ so}} \cr & \int_e^\infty {\frac{{\ln x}}{x}} dx = \mathop {\lim }\limits_{b \to \infty } \int_1^b {\ln x\left( {\frac{1}{x}} \right)} dx \cr & {\text{Integrate, }}\int {{u^n}} du = \frac{{{u^{n + 1}}}}{{n + 1}} + C \cr & \mathop {\lim }\limits_{b \to \infty } \int_1^b {\ln x\left( {\frac{1}{x}} \right)} dx = \mathop {\lim }\limits_{b \to \infty } \left[ {\frac{{{{\left( {\ln x} \right)}^2}}}{2}} \right]_1^b = \frac{1}{2}\mathop {\lim }\limits_{b \to \infty } \left[ {{{\left( {\ln x} \right)}^2}} \right]_1^b \cr & \frac{1}{2}\mathop {\lim }\limits_{b \to \infty } \left[ {{{\left( {\ln x} \right)}^2}} \right]_1^b = \frac{1}{2}\mathop {\lim }\limits_{b \to \infty } \left[ {{{\left( {\ln b} \right)}^2} - {{\left( {\ln 1} \right)}^2}} \right] \cr & \frac{1}{2}\mathop {\lim }\limits_{b \to \infty } \left[ {{{\left( {\ln b} \right)}^2} - 0} \right] \cr & {\text{Evaluate the limit}} \cr & \frac{1}{2}\mathop {\lim }\limits_{b \to \infty } \left[ {{{\left( {\ln b} \right)}^2}} \right] = \frac{1}{2}\left[ {{{\left( {\ln \infty } \right)}^2}} \right] \cr & {\text{Simplify}} \cr & \frac{1}{2}\left[ \infty \right] \cr & \infty \cr & {\text{Therefore,}} \cr & {\text{The improper integral diverges}} \cr} $$
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