Differential Equations and Linear Algebra (4th Edition)

Published by Pearson
ISBN 10: 0-32196-467-5
ISBN 13: 978-0-32196-467-0

Chapter 1 - First-Order Differential Equations - 1.6 First-Order Linear Differential Equations - Problems - Page 59: 13

Answer

\[y(x)=\frac{1}{2}x^3(2\ln x-1)+Cx\]

Work Step by Step

$y'-x^{-1}y=2x^2\ln x$ ___(1) $\frac{dy}{dx}-\frac{1}{x}y=2x^2\ln x$ ____(2) This is linear differential equation Integrating Factor:- \[I(x)=e^{\int -\frac{1}{x}dx}=e^{-\ln|x|}=\frac{1}{x}\] Multiply (2) by $I(x)$ $\frac{1}{x}\frac{dy}{dx}-\frac{1}{x^2}y=2x\ln x$ $\frac{d}{dx}\left(\frac{y}{x}\right)=2x\ln x$ Integrating, $\frac{y}{x}=2\int x\ln xdx+C$ $C$ is constant of integration $\frac{y}{x}=2\left[\ln x.\frac{x^2}{2}-\int\frac{1}{x}.\frac{x^2}{2}dx\right]+C$ $\frac{y}{x}=2\left[\frac{x^2}{2}\ln x-\frac{x^2}{4}\right]+C$ $y=x^3\left[\ln x-\frac{1}{2}\right]+Cx$ $y=\frac{1}{2}x^3(2\ln x-1)+Cx$ Hence general solution of (1) is $y(x)=\frac{1}{2}x^3(2\ln x-1)+Cx$.
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