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 - 1.8 Change of Variables - Problems - Page 80: 42

Answer

$y=\frac{1}{2x^5-Cx^2}$

Work Step by Step

Given: $$y'+2x^{-1}y=6y^2x^4$$ This equation is in form of Bernoulli equation, where $p(x)=2x^{-1}, q(x)=6y^2x^4$ and $n=2$ $$\frac{dy}{dx}+2x^{-1}y=6y^2x^4$$ Dividing both sides by $y^2$ $$\frac{1}{y^2}\frac{du}{dx}+\frac{2}{xy}=6x^4$$ Let $u=y^{-1} \rightarrow \frac{du}{dx}=-\frac{1}{2y^2}\frac{dy}{dx}$ Then it becomes: $$-\frac{du}{dx}+\frac{2u}{x}=6x^4$$ $$\frac{du}{dx}-\frac{2u}{x}=-6x^4$$ The integrating factor is: $$I(x)=e^{\int -\frac{2}{x}dx}=e^{-2\ln (x)}=x^{-2}$$ The equation becomes: $$\frac{d}{dx}(ux^{-2})=x^{-2}\frac{du}{dx}-2x^{-3}u=-6x^4x^{-2}=-6x^2$$ Integrating both sides: $$ux^{-2}=-6\frac{x^3}{3}+C$$ $$u=-2x^5+Cx^2$$ where $C$ is a constant of integration Subtitute when $u=y^{-1}$ we have: $$\rightarrow y^{-1}=-2x^5+Cx^2$$ $$\rightarrow y=\frac{1}{2x^5-Cx^2}$$
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