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: 40

Answer

See below

Work Step by Step

Given: $\frac{dy}{dx}-\frac{3}{2x}y=6y^{\frac{1}{3}}x^2\ln x$ This is a Bernoulli equation, where $p(x)=-\frac{3}{2x}\\q(x)=6x^2\ln x\\n=\frac{1}{3}$ Divide both sides of the given equation by $y^{\frac{1}{3}}$, we have: $^{-\frac{1}{3}}\frac{dy}{dx}-{\frac{3}{2x}}y^{\frac{2}{3}}=6x^2\ln x$ To solve this, we let $u=y^{\frac{2}{3}} \rightarrow \frac{du}{dx}=\frac{2}{3}y^{-\frac{1}{3}}\frac{dy}{dx}$ the equation then becomes: $\frac{3}{2}\frac{du}{dx}-\frac{3}{2x}u=6x^2\ln x\\\frac{du}{dx}-\frac{1}{x}u=4x^2\ln x$ Integrating factor: $I(x)=e^{-\int \frac{1}{x}dx}=e^{-\ln|x|}=|x|^{-1}$ Hence, $\frac{d}{dx}(\frac{u}{x})=-\frac{1}{x^2}u+\frac{1}{x}\frac{du}{dx}\\ \rightarrow \frac{d}{dx}(\frac{u}{x})=4x\ln x$ Integrate: $\int \frac{d}{dx}(\frac{u}{x})=\int 4x\ln x$ Let $u=\ln x \rightarrow du=\frac{1}{x}dx\\dv=xdx \rightarrow v=\frac{x^2}{2}$ then $\frac{u}{x}=\int 4x \ln x\\ \rightarrow \frac{u}{x}=4(\frac{x^2}{2}\ln x-\int \frac{x^2}{2}\frac{1}{x}du)\\ \rightarrow \frac{u}{x}=2x^2\ln x-x^2\\ \rightarrow u=2x^3\ln x-x^3$ Substitute: $y^{\frac{2}{3}}=2x^3\ln x-x^3$ Hence, $y^{\frac{2}{3}}=x^3(2\ln x-1)+c$ with $c$ is a constant.
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