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 - 1.12 Chapter Review - Additional Problems - Page 110: 39

Answer

$y=\sqrt \frac{25(\ln x)^2+C}{2x^2}$

Work Step by Step

We are given: $$\frac{dy}{dx}+\frac{y}{x}=\frac{25\ln x}{2x^3y}$$ Dividing both sides by $y^2$: $$y\frac{dy}{dx}+\frac{1}{x}y^2=\frac{25\ln x}{2x^3}$$ Assume that $u=y^2\rightarrow \frac{du}{dx}=2y\frac{dy}{dx}$ The equation becomes: $$\frac{1}{2}\frac{du}{dx}+\frac{1}{x}u=\frac{25\ln x}{2x^3}$$ $$\frac{du}{dx}+\frac{2}{x}u=\frac{25 \ln x}{x^3}$$ Itegrating factor: $$I=e^{\int \frac{2}{x}dx}=e^{2\ln x}=x^2$$ The equation becomes: $$\frac{d}{dx} (ux^2)=25x^{-1}\ln x$$ Integrating both sides: $$ux^2=\frac{25}{2}(\ln x)^2+C$$ $$y^2=\frac{\frac{25}{2}(\ln x)^2+C}{x^2}$$ The general solution is $y=\sqrt \frac{25(\ln x)^2+C}{2x^2}$
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