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 645: 20

Answer

$\displaystyle y = (1)e^{x^2/2}+1$

Work Step by Step

$\displaystyle f'(x)=xf(x)-x$ is the same thing as $y'=xy-x$ $\displaystyle \frac{dy}{dx} = x(y-1)$ Separation of variables: $\displaystyle \frac{dy}{y-1} = x\cdot dx$ Integrate with respect to each variable $\displaystyle \int\frac{dy}{y-1} = \int x\cdot dx$ $\displaystyle \ln|y-1|=\frac{x^2}{2}+C$ $e^{\ln|y-1|} = e^{x^2/2+C} = e^C\cdot e^{x^2/2}$ $\displaystyle y-1 = Ce^{x^2/2}$ The reason that $e^C$ can be expressed as just $C$ is because $C$ has not taken on an initial value yet. Now plug in the initial condition and solve for $C$ $\displaystyle (2)-1 = Ce^{(0)^2/2}$ $\displaystyle 1 = C(1) = C$ Plug the value for $C$ that we just found back into the equation and isolate $y$ $\displaystyle y-1 = (1)e^{x^2/2}$ $\displaystyle y = (1)e^{x^2/2}+1$
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