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: 15

Answer

$y = \frac{ln~x}{x}-\frac{1}{x}+\frac{3}{x^2}$

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: $x^2y' + 2xy = ln~x$ $y' + \frac{2}{x}y = \frac{ln~x}{x^2}$ Let $P(x) = \frac{2}{x}$ Let $Q(x) = \frac{ln~x}{x^2}$ We can find the integrating factor: $I(x) = e^{\int \frac{2}{x} ~dx} = e^{2~ln~x} = e^{ln~x^2} = x^2$ We can multiply by the integrating factor: $x^2~y'+2x~y = ln~x$ $\frac{d}{dx}(x^2~y) = ln~x$ We can integrate both sides of the equation: $x^2~y = \int ln~x~dx$ $x^2~y = x~ln~x-x+C$ $y = \frac{ln~x}{x}-\frac{1}{x}+\frac{C}{x^2}$ We can use the initial condition to find $C$: $y(1) = 2$ $\frac{ln~1}{1}-\frac{1}{1}+\frac{C}{1^2} = 2$ $0-1+C = 2$ $C = 3$ We can write the solution: $y = \frac{ln~x}{x}-\frac{1}{x}+\frac{3}{x^2}$
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