Calculus (3rd Edition)

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

Chapter 8 - Techniques of Integration - 8.2 Trigonometric Integrals - Exercises - Page 404: 64

Answer

\begin{align*} \int \sin ^{2} x \cos ^{3} x d x &= \frac{1}{3}\sin^3 x-\frac{1}{5}\sin^5x+C\\ \int \sin ^{2} x \cos ^{3} x d x &= \frac{1}{30}(7+3 \cos 2 x) \sin ^{3} x+C \end{align*} These are equivalent.

Work Step by Step

Since \begin{align*} \int \sin ^{2} x \cos ^{3} x d x&=\int \sin ^{2} x \cos ^{2} x\cos x d x\\ &=\int \sin ^{2} x (1-\sin ^{2} x)\cos x d x\\ &=\int ( \sin ^{2} x-\sin ^{4} x)\cos x d x\\ &= \frac{1}{3}\sin^3 x-\frac{1}{5}\sin^5x+C \end{align*} and \begin{align*} \frac{1}{3}\sin^3 x-\frac{1}{5}\sin^5x&=\left( \frac{1}{3}-\frac{1}{5}\sin^2x \right)\sin^3 x\\ &= \frac{1}{15}\left( 5-3\sin^2x \right)\sin^3 x\\ &= \frac{1}{15}\left( 5-\frac{3}{2}(1-\cos 2x)\right)\sin^3 x\\ &= \frac{1}{30}\left( 10-3(1-\cos 2x)\right)\sin^3 x\\ &=\frac{1}{30}(7+3 \cos 2 x) \sin ^{3} x \end{align*} Hence \begin{align*} \int \sin ^{2} x \cos ^{3} x d x &= \frac{1}{3}\sin^3 x-\frac{1}{5}\sin^5x+C\\ \int \sin ^{2} x \cos ^{3} x d x &= \frac{1}{30}(7+3 \cos 2 x) \sin ^{3} x+C \end{align*} are equivalent to each other.
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