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 79: 26

Answer

$(2y-x)(y+x)=2$

Work Step by Step

Given: $$\frac{dy}{dx}=\frac{2(2y-x)}{x+y}$$ Let $y=vx \rightarrow \frac{dy}{dx}=v+x\frac{dv}{dx}$ Then it becomes: $$v+x\frac{dv}{dx}=\frac{2-v}{1+4v}$$ $$x\frac{dv}{dx}=-\frac{2(v^2+v-1)}{4v+1}$$ $$\frac{4v+1}{-4v^2-2v+2}dv=\frac{1}{x}dx$$ Integrating both sides: $\int \frac{4v+1}{-4v^2-2v+2}dv=\int \frac{1}{x}dx$ $$\int (-\frac{1}{2v-1}-\frac{1}{2v+2})dv=\int \frac{1}{x}dx$$ $$\frac{-1}{2}\ln(2v-1)-\frac{1}{2}\ln(v+1)=\ln x +\ln C$$ where $C$ is a constant of integration. $$(2v-1)-1)(v+1)x^2=C$$ Subtitute when $y=vx $ $$\rightarrow (2y-x)(y+x)=C$$ Since $y(1)=1$ $$\rightarrow (2\times1-1)(1+1)=C$$ $$C=2$$ The solution to the differential equation is: $$(2y-x)(y+x)=2$$
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