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 - 8.6 Exercises - Page 555: 31

Answer

The solution is $$\int\frac{\ln x}{x(3+2\ln x)}dx=\frac{1}{4}(3+2\ln x)-\frac{3}{4}\ln|3+2\ln x|+c.$$

Work Step by Step

To solve the integral $$\int\frac{\ln x}{x(3+2\ln x)}dx$$ we will use substitution $\ln x=t$ which gives us $\frac{dx}{x}=dt$. Putting this into the integral we have: $$\int\frac{\ln x}{x(3+2\ln x)}dx=\int\frac{t}{3+2t}dt.$$ Now we will use new substitution $z=3+2t\Rightarrow\frac{dz}{2}=dt,t=\frac{z-3}{2}.$ Using this we get: $$\int\frac{t}{3+2t}dt=\frac{1}{2}\int\frac{\frac{z-3}{2}}{z}dz=\frac{1}{4}\int\frac{z-3}{z}dz=\frac{1}{4}\int dz-\frac{3}{4}\int\frac{dz}{z}=\frac{1}{4}z-\frac{3}{4}\ln|z|+c.$$ We will express $z$ in terms of $t$ and then $t$ in terms of $x$: $$\frac{1}{4}z-\frac{3}{4}\ln|z|+c=\frac{1}{4}(3+2t)-\frac{3}{4}\ln|3+2t|+c=\frac{1}{4}(3+2\ln x)-\frac{3}{4}\ln|3+2\ln x|+c.$$
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