Calculus: Early Transcendentals 8th Edition

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

Chapter 9 - Section 9.5 - Linear Equations - 9.5 Exercises - Page 625: 19

Answer

$y = -x~cos~x-x$

Work Step by Step

A first-order linear differential equation is one that can be put into the form: $\frac{dy}{dx} + P(x)~y = Q(x)$ We can consider the given equation: $xy' = y + x^2~sin~x$ $y' - \frac{y}{x} = x~sin~x$ Let $P(x) = -\frac{1}{x}$ Let $Q(x) = x~sin~x$ We can find the integrating factor: $I(x) = e^{\int -\frac{1}{x} ~dx} = e^{-ln~x} = e^{ln~x^{-1}} = \frac{1}{x}$ We can multiply by the integrating factor: $\frac{1}{x}y'-\frac{y}{x^2} = sin~x$ $\frac{d}{dx}(\frac{y}{x}) = sin~x$ We can integrate both sides of the equation: $\frac{y}{x} = \int sin~x~dx$ $\frac{y}{x} = -cos~x+C$ $y = -x~cos~x+C~x$ We can use the initial condition to find $C$: $y(\pi) = 0$ $-\pi~cos~\pi+C~\pi = 0$ $-\pi~(-1)+C~\pi = 0$ $\pi+C~\pi = 0$ $C~\pi = -\pi$ $C = -1$ We can write the solution: $y = -x~cos~x-x$
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