Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 9 - Review - Exercises - Page 635: 11

Answer

The solution of the given initial-value problem $$ x y^{\prime}-y=x \ln x \quad\quad y(1)=2 $$ is $$ y=\frac{1}{2} x(\ln x)^{2}+2 x $$

Work Step by Step

The initial-value problem is: $$ x y^{\prime}-y=x \ln x \quad\quad y(1)=2 $$ $$ x y^{\prime}-y=x \ln x \Rightarrow y^{\prime}-\frac{1}{x} y=\ln x \quad\quad (1) $$ This is a linear equation and the integrating factor is $$ I(x)=e^{\int(-1 / x) d x}=e^{-\ln |x|}=\left(e^{\ln |x|}\right)^{-1}=|x|^{-1}=1 / x $$ Multiplying Eq.(1) by $1 / x $ gives $$ \frac{1}{x} y^{\prime}-\frac{1}{x^{2}} y=\frac{1}{x} \ln x \Rightarrow\left(\frac{1}{x} y\right)^{\prime}=\frac{1}{x} \ln x $$ Then we integrate both sides of the last equation: $$ \frac{1}{x} y=\int \frac{\ln x}{x} d x \Rightarrow \frac{1}{x} y=\frac{1}{2}(\ln x)^{2}+C $$ So the solution of the differential equation is: $$ y=\frac{1}{2} x(\ln x)^{2}+C x $$ Now $$ y(1)=2 \Rightarrow 2=0+C \Rightarrow C=2, $$ Thus: $$ y=\frac{1}{2} x(\ln x)^{2}+2 x $$
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