Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 8 - Techniques of Integration - 8.1 Integration by Parts - Exercises - Page 395: 45

Answer

$$\int \frac{ \ln(\ln x) \ln x}{x} d x=\frac{1}{2}(\ln x)^2\ln (\ln x)-\frac{1}{4} (\ln x)^2+c$$

Work Step by Step

Let $u=\ln x$, then $du=\frac{1}{ x}dx$. Hence, we have $$\int \frac{ \ln(\ln x) \ln x}{x} d x= \int u \ln u \ du .$$ Now we rewrite with the variable z to avoid confusion with variable names and use integration by parts as follows; let $u\ln z$ and $dv=u$: $$ \int z \ln z \ dz=\int u dv= uv-\int vdu\\ =\frac{1}{2}z^2\ln z-\frac{1}{2}\int z dz\\ =\frac{1}{2}z^2\ln z-\frac{1}{4} z^2 .$$ Hence $$\int \frac{ \ln(\ln x) \ln x}{x} d x=\frac{1}{2}(\ln x)^2\ln (\ln x)-\frac{1}{4} (\ln x)^2+c$$
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