Calculus, 10th Edition (Anton)

Published by Wiley
ISBN 10: 0-47064-772-8
ISBN 13: 978-0-47064-772-1

Chapter 7 - Principles Of Integral Evaluation - 7.5 Integrating Rational Functions By Partial Fractions - Exercises Set 7.5 - Page 521: 21

Answer

$$\frac{{{x^3}}}{3} + x - 2\ln \left| x \right| + \ln \left| {x + 1} \right| + 2\ln \left| {x - 1} \right| + C$$

Work Step by Step

$$\eqalign{ & \int {\frac{{{x^5} + {x^2} + 2}}{{{x^3} - x}}} dx \cr & {\text{The integrand is an improper rational function since the numerator }} \cr & {\text{has degree 5 and the denominator 3}}{\text{. Thus}}{\text{, using the long division}} \cr & \,\,\,\,\,\,\frac{{{x^5} + {x^2} + 2}}{{{x^3} - x}} = {x^2} + 1 + \frac{{{x^2} + x + 2}}{{{x^3} - x}} \cr & {\text{The integrand can be expressed as}} \cr & \int {\left( {{x^2} + 1 + \frac{{{x^2} + x + 2}}{{{x^3} - x}}} \right)} dx \cr & = \frac{{{x^3}}}{3} + x + \int {\frac{{{x^2} + x + 2}}{{{x^3} - x}}} dx \cr & {\text{Decomposing the integrand into partial fractions}} \cr & {\text{Factor the denominator}} \cr & \frac{{{x^2} + x + 2}}{{{x^3} - x}} = \frac{{{x^2} + x + 2}}{{x\left( {x + 1} \right)\left( {x - 1} \right)}} \cr & \frac{{{x^2} + x + 2}}{{x\left( {x + 1} \right)\left( {x - 1} \right)}} = \frac{A}{x} + \frac{B}{{x + 1}} + \frac{C}{{x - 1}} \cr & {\text{Multiplying the equation by }}\left( {x - 2} \right)\left( {x - 1} \right){\text{, we have}} \cr & {x^2} + x + 2 = A\left( {x + 1} \right)\left( {x - 1} \right) + Bx\left( {x - 1} \right) + Cx\left( {x + 1} \right) \cr & {\text{if we set }}x = 0 \cr & 2 = A\left( 1 \right)\left( { - 1} \right) + B\left( 0 \right) + C\left( 0 \right) \cr & A = - 2 \cr & {\text{if we set }}x = - 1 \cr & 1 - 1 + 2 = A\left( 0 \right) + B\left( { - 1} \right)\left( { - 2} \right) + C\left( 0 \right) \cr & 2 = B\left( 2 \right) \cr & B = 1 \cr & {\text{if we set }}x = 1 \cr & 1 + 1 + 2 = A\left( 0 \right) + B\left( 0 \right) + C\left( 1 \right)\left( {1 + 1} \right) \cr & 4 = C\left( 2 \right) \cr & C = 2 \cr & {\text{then}} \cr & \frac{{{x^2} + x + 2}}{{x\left( {x + 1} \right)\left( {x - 1} \right)}} = \frac{{ - 2}}{x} + \frac{1}{{x + 1}} + \frac{2}{{x - 1}} \cr & \cr & \frac{{{x^3}}}{3} + x + \int {\frac{{{x^2} + x + 2}}{{{x^3} - x}}} dx = \frac{{{x^3}}}{3} + x + \int {\left( {\frac{{ - 2}}{x} + \frac{1}{{x + 1}} + \frac{2}{{x - 1}}} \right)} dx \cr & {\text{Integrating}} \cr & = \frac{{{x^3}}}{3} + x - 2\ln \left| x \right| + \ln \left| {x + 1} \right| + 2\ln \left| {x - 1} \right| + C \cr} $$
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