Calculus with Applications (10th Edition)

Published by Pearson
ISBN 10: 0321749006
ISBN 13: 978-0-32174-900-0

Chapter 8 - Further Techniques and Applications of Integration - 8.4 Improper Integrals - 8.4 Exercises - Page 452: 23

Answer

$${\text{The integral is divergent}}$$

Work Step by Step

$$\eqalign{ & \int_2^\infty {\frac{1}{{x\ln x}}} dx \cr & {\text{by the definition of an improper integral}}{\text{}} \cr & \int_2^\infty {\frac{1}{{x\ln x}}} dx = \mathop {\lim }\limits_{b \to \infty } \int_2^b {\frac{1}{{x\ln x}}} dx \cr & \cr & {\text{integrating }}\int {\frac{1}{{x\ln x}}} dx \cr & {\text{set }}u = \ln x{\text{ then }}\frac{{du}}{{dx}} = \frac{1}{x},\,\,\,\,xdu = dx \cr & \int {\frac{1}{{x\ln x}}} dx = \int {\frac{1}{{xu}}} \left( {xdu} \right) = \int {\frac{1}{u}} du \cr & = \ln \left| u \right| + C \cr & {\text{replace }}u = \ln x \cr & = \ln \left| {\ln x} \right| + C \cr & {\text{then}} \cr & \mathop {\lim }\limits_{b \to \infty } \int_2^b {\frac{1}{{x\ln x}}} dx = \mathop {\lim }\limits_{b \to \infty } \left( {\ln \left| {\ln x} \right|} \right)_2^b \cr & = \mathop {\lim }\limits_{a \to - \infty } \left( {\ln \left| {\ln b} \right| - \ln \left| {\ln 2} \right|} \right) \cr & {\text{evaluating the limit when }}b \to \infty \cr & = \ln \left| {\ln \infty } \right| - \ln \left| {\ln 2} \right| \cr & = \infty \cr & then \cr & {\text{The integral is divergent}} \cr} $$
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