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: 47

Answer

$y=\sqrt \frac{\ln x}{x^2(1-2\ln x)+C}$

Work Step by Step

Given: $$\frac{dy}{dx}-\frac{1}{2x \ln x}y=2xy^3$$ This equation is in form of Bernoulli equation, where $p(x)=-\frac{1}{2x \ln x}, q(x)=2x$ and $n=3$ Dividing both sides by $y^3$ $$\frac{1}{y^3}\frac{dy}{dx}-\frac{1}{2x \ln x}\frac{1}{y^2}=2x$$ Let $u=y^{-2} \rightarrow \frac{du}{dx}=-2\frac{1}{y^3}\frac{dy}{dx}$ Then it becomes: $$-\frac{1}{2}\frac{du}{dx}-\frac{u}{2x \ln x}=2x$$ $$\frac{du}{dx}+\frac{u}{x \ln x}=-4x$$ The integrating factor is: $$I(x)=e^{\int \frac{1}{x \ln x}dx}$$ To solve this, we should let $u=\ln x \rightarrow du=\frac{1}{x}dx \rightarrow \int \frac{1}{x \ln x}dx=\int \frac{1}{u}du = \ln (u)=\ln (\ln (x))$ Hence, the integrating factor is: $$I(x)=e^{\ln (\ln (x))}=\ln x$$ The equation becomes: $$\frac{d}{dx}(u \ln x)=\frac{1}{x}u+\ln x\frac{du}{dx}$$ $$\frac{d}{dx}(u \ln x)=-4x \ln x$$ Integrating both sides: $$\int \frac{d}{dx}(u \ln x)=-4\int x \ln x dx$$ To solve the right side, we let $u=\ln x, dv=xdx$ $\rightarrow du=\frac{1}{x}dx, v=\frac{x^2}{2}$ $$-4\int x \ln xdx=-4(\frac{x^2}{2}\ln x -\int \frac{x}{2}dx)=-4(\frac{x^2}{2}\ln x-\frac{x^2}{4}+C)$$ where $C$ is a constant of integration Then: $$u \ln x =-4(\frac{x^2}{2}\ln x-\frac{x^2}{4}+C)=-2x^2 \ln x+x^2 +C$$ $$u=\frac{x^2}{\ln x}-2x^2+\frac{C}{\ln x}$$ Subtitute when $u=y^{-2}$ we have: $$\rightarrow y^{-2}=\frac{x^2}{\ln x}-2x^2+\frac{C}{\ln x}$$ $$\rightarrow y=\sqrt \frac{\ln x}{x^2(1-2\ln x)+C}$$
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