Calculus 8th Edition

Published by Cengage
ISBN 10: 1285740629
ISBN 13: 978-1-28574-062-1

Chapter 9 - Differential Equations - 9.3 Separable Equations - 9.3 Exercises - Page 646: 34

Answer

$y=\sqrt{2(\ln(x)+2)}$

Work Step by Step

Using the FTC it follows: $$y'=\frac{1}{xy}$$ $$ydy=\frac{1}{x}dx$$ $$\int ydy=\int \frac{1}{x}dx$$ $$\frac{y^{2}}{2}=\ln(x)+C$$ $$y^{2}=2(\ln(x)+C)$$ From the given equation for $x=1$, $y=2+0=2$ $$y^{2}=2(\ln(x)+C)$$ $$2^{2}=2(\ln(1)+C) \to C=2$$ $$y^{2}=2(\ln(x)+2)$$ $$y=\sqrt{2(\ln(x)+2)}~~\text{or}~~y=-\sqrt{2(\ln(x)+2)}$$ The equation $y=-\sqrt{2(\ln(x)+2)}$ should be discarded because when $x=1$, the value of $y$ is $-2$ which is not adequat with the initial condition. So the solution is: $$y=\sqrt{2(\ln(x)+2)}$$
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