Calculus 8th Edition

Published by Cengage
ISBN 10: 1285740629
ISBN 13: 978-1-28574-062-1

Chapter 7 - Techniques of Integration - 7.5 Strategy for Integration - 7.5 Exercises - Page 548: 72

Answer

$$ -\frac{\ln (x+1)}{x}+ \ln (x)-\ln(x+1)+c$$

Work Step by Step

Given $$\int \frac{\ln (x+1)}{x^2} d x$$ Use integration by parts \begin{aligned} u&= \ln (x+1) \ \ \ \ \ \ dv= x^{-2}dx\\ du&= \frac{dx}{x+1}\ \ \ \ \ \ \ \ \ v= \frac{-1}{x} \end{aligned} Then \begin{aligned} \int \frac{\ln (x+1)} {x^2} d x&= -\frac{\ln (x+1)}{x}+\int \frac{dx}{x(x+1)} \end{aligned} Use partial fractions \begin{aligned} \frac{1}{x\left(x+1\right)}&=\frac{a_0}{x}+\frac{a_1}{x+1}\\ 1&= a_0\left(x+1\right)+a_1x \end{aligned} at $x=0\to\ \ a_0 = 1,\ \ x=-1\to\ \ a_0 = -1$ Then \begin{aligned} \int \frac{dx}{x(x+1)}&= \int \frac{1}{x}dx-\int \frac{1}{x+1}dx\\ &= \ln (x)-\ln(x+1)+c \end{aligned} Hence \begin{aligned} \int \frac{\ln (x+1)} {x^2} d x&= -\frac{\ln (x+1)}{x}+ \ln (x)-\ln(x+1)+c \end{aligned}
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