Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 7 - Section 7.4 - Integration of Rational Functions by Partial Fractions. - 7.4 Exercises - Page 501: 31

Answer

$ \displaystyle \frac{1}{3}\ln|x-1| -\frac{1}{6}\cdot\ln|x^{2}+x+1|-\frac{1}{\sqrt{3}}\arctan(\frac{2x+1}{\sqrt{3}})+C$

Work Step by Step

$x^{3}-1$ is a difference of cubes $\displaystyle \frac{1}{(x-1)(x^{2}+x+1)}=\frac{A}{x-1}+\frac{Bx+C}{x^{2}+x+1}$ $1=A(x^{2}+x+1)+(Bx+C)(x-1)$ $1=Ax^{2}+Ax+A+Bx^{2}-Bx+Cx-C$ $1=(A+B)x^{2}+(A-B+C)x+(A-C)$ $A+B=0\Rightarrow A=-B$ $A-C=1\Rightarrow C=A-1$ $A-B+C=0\displaystyle \Rightarrow A+A+A-1=0\Rightarrow A=\frac{1}{3}$ $B=-\displaystyle \frac{1}{3},C=-\frac{2}{3}$ $\displaystyle \frac{1}{(x-1)(x^{2}+x+1)}=\frac{\frac{1}{3}}{x-1}+\frac{-\frac{1}{3}x-\frac{2}{3}}{x^{2}+x+1}$ $=\displaystyle \frac{1}{3(x-1)} -\frac{x+2}{3(x^{2}+x+1)}$ $=\displaystyle \frac{1}{3(x-1)} -\frac{1}{3}\cdot\frac{\frac{1}{2}(2x+1)+\frac{3}{2}}{ x^{2}+x+1}$ $=\displaystyle \frac{1}{3(x-1)} -\frac{1}{6}\cdot\frac{2x+1}{ x^{2}+x+1} -\frac{1}{3}\cdot\frac{\frac{3}{2}}{ x^{2}+x+(\frac{1}{2})^{2}+\frac{3}{4}}$ $=\displaystyle \frac{1}{3(x-1)} -\frac{1}{6}\cdot\frac{2x+1}{ x^{2}+x+1} -\frac{1}{2}\cdot\frac{1}{ (x+\frac{1}{2})^{2}+(\frac{\sqrt{3}}{2})^{2}}$ $\displaystyle \int\frac{1}{x^{3}-1}dx=$ $\displaystyle \frac{1}{3}\ln|x-1| -\frac{1}{6}\cdot\ln|x^{2}+x+1|-\frac{1}{\sqrt{3}}\arctan(\frac{2x+1}{\sqrt{3}})+C$
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