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: 43

Answer

$\rightarrow y=\frac{1}{\sqrt x^3+Cx}$

Work Step by Step

Given: $$2x(y'+y^3x^2)+y=0$$ $$2x\frac{dy}{dx}+y=-2y^3x^3$$ $$\frac{dy}{dx}+\frac{y}{2x}=-y^3x^2$$ This equation is in form of Bernoulli equation, where $p(x)=\frac{1}{2x}, q(x)=-x^2$ and $n=3$ Dividing both sides by $y^3$ $$\frac{1}{y^3}\frac{dy}{dx}+\frac{1}{2xy^2}=-x^2$$ Let $u=y^{-2} \rightarrow \frac{du}{dx}=-\frac{2}{y^3}\frac{dy}{dx}$ Then it becomes: $$-\frac{du}{2dx}+\frac{u}{2x}=-x^2$$ $$\frac{du}{dx}-\frac{u}{x}=2x^2$$ The integrating factor is: $$I(x)=e^{\int -\frac{1}{x}dx}=e^{-\ln (x)}=x^{-1}$$ The equation becomes: $$\frac{d}{dx}(ux^{-1})=x^{-1}\frac{du}{dx}-x^{-2}u=2x$$ Integrating both sides: $$ux^{-1}=x^2+C$$ $$u=x^3+Cx$$ where $C$ is a constant of integration Subtitute when $u=y^{-2}$ we have: $$\rightarrow y^{-2}=x^3+Cx$$ $$\rightarrow y=\frac{1}{\sqrt x^3+Cx}$$
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