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: 32

Answer

$y=|x|\sqrt Cx-1$

Work Step by Step

We are given: $$(3y^2+x^2)dx-2xydy=0$$ We have that: $$M(x,y)=3y^2+x^2\wedge N(x,y)=-2xy$$ then $$M'(x,y)=6y$$ $$N'(x,y)=-2y$$ So this equation is not exist. But we have: $$\frac{M'_y-N'_y}{M}=\frac{8y}{-2xy}=-\frac{4}{x}$$ Integrating factor: $$I=e^{-\int -\frac{4}{x}dx}=e^{-4\ln x}=x^{-4}$$ Multiplying the equation by $x^{-4}$ $$(3y^2x^{-4}+x^2)dx-2x^{-3}ydy=0$$ There exists a potential function $\phi$ satisfies: $$\frac{\partial \phi}{\partial x}=M \wedge \frac{\partial \phi}{\partial y}=N$$ $$\frac{\partial \phi}{\partial x}=3y^2x^{-4}+x^2 \wedge \frac{\partial \phi}{\partial y}=-2x^{-3}y$$ Integrating $\frac{\partial \phi}{\partial x}=3y^2x^{-4}+x^2$ we get $$\phi=-y^2x^{-3}-x^{-1}+f(y)$$ Since $\frac{\partial \phi}{\partial y}=-2yx^{-3}+f'(y)$ We obtain: $$f'(y)=0\rightarrow f(y)=C$$ $C$ is constant of integration The potential solution is: $$\phi=-y^2x^{-3}+x^{-1}$$ $$y^2=Cx^3-x^2$$ Hence general solution is $y=|x|\sqrt Cx-1$
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