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 82: 65

Answer

$y(x)=xe^{x^2}$

Work Step by Step

We are given: $$y^{-1}y'-2x^{-1}y\ln y=x^{-1}(1-2\ln x)$$ Let $t= \ln y \rightarrow t'=y^{-1}y$ Substituting: $$t'-2x^{-1}t=x^{-1}(1-2 \ln x)$$ We have $p(x)=-2x^{-1} q(x)=x^{-1}(1-2\ln x)$ The solution can be found by obtaning: $$t=e^{-\int p(x)dx}[\int e^{p(x)dx}q(x)dx+C$$ $$t=e^{2x^{-1}dx}[\int e^{-2x^{-1}dx}x^{-1}1-2\ln x)dx+C$$ $$t=e^{2\ln x}[\int e^{-2\ln x}x^{-1}(1-2\ln x)dx + C$$ $$t=x^2[\int (x^{-3}-2x^{-3}\ln x)dx+C$$ $$t=x^2[\frac{x^{-2}}{-2}+\frac{\ln x}{x^2}+\frac{x^{-2}}{2}+C$$ $$t=\ln x +Cx^2$$ Since $t= \ln y\rightarrow y=e^t$ Hence $y=e^{\ln x+Cx^2}=xe^{Cx^2}$ We are given: $y(1)=e$ Substituting: $1e^C=e \rightarrow C=1$ The solution is: $$y(x)=xe^{x^2}$$
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