Multivariable Calculus, 7th Edition

Published by Brooks Cole
ISBN 10: 0-53849-787-4
ISBN 13: 978-0-53849-787-9

Chapter 17 - Second-Order Differential Equations - 17.2 Exercises - Page 1179: 9

Answer

$y=2e^{x}+\frac{1}{2}x^{2}e^{x}-xe^{x}$

Work Step by Step

$y''-y'=$ Use auxiliary equation $r^{2}-r=0$ $r(r-1)=0$ $r_{1}=0$ $r_{2}=1$ $y_{c}=c_{1}e^{r_{1}x}+c_{2}e^{r_{2}x}$ $y_{c}=c_{1}+c_{2}e^{x}$ The particular solution is of the form $y_{p}=x(A+Bx)e^{x}$ It is not $y_{p}=Ae^{x}+Bxe^{x}$ because the coefficient of $xe^{x}$ is the same in the expression of $y''$ and $y'$ and when we substitute the expressions of $y''$ and $y'$ in the main equation, we get coefficient of $xe^{x}$ as $0$. Assuming that $y_{p}=Axe^{x}+Bx^{2}e^{x}$ $y_{p}'=(Axe^{x}+Ae^{x})+(Bx^{2}e^{x}+2Bxe^{x})$ $y_{p}'=Axe^{x}+xe^{x}(A+2B)+Bx^{2}e^{x}$ $y_{p}''=Ae^{x}+[xe^{x}(A+2B)+e^{x}(A+2B)]+[Bx^{2}e^{x}+2Bxe^{x}]$ $y_{p}''=(2A+2B)e^{x}+xe^{x}(A+4B)+Bx^{2}e^{x}$ Substitute into the main differential equation $((2A+2B)e^{x}+xe^{x}(A+4B)+Bx^{2}e^{x})-(Ae^{x}+xe^{x}(A+2B)+Bx^{x}e^{x})=xe^{x}$ $(A+2B)e^{x}+xe^{x}(2B)=xe^{x}$ $2B=1$ $B=\frac{1}{2}$ $A+2B=0$ $A=-2B$ $A=-2(\frac{1}{2})$ $A=-1$ Therefore $y_{p}=\frac{1}{2}x^{2}e^{x}-xe^{x}$ $y=y_{c}+y_{p}$ $y=c_{1}+c_{2}e^{x}+\frac{1}{2}x^{2}e^{x}-xe^{x}$ First condition $y(0)=2$ $2=c_{1}+c_{2}e^{0}+\frac{1}{2}(0)^{2}e^{0}-(0)e^{0}$ $2=c_{1}+c_{2}$ Second condition $y'(0)=1$ $y'=c_{2}e^{x}+(\frac{1}{2}x^{2}e^{x}+xe^{x})-(xe^{x}+e^{x})$ $1=c_{2}e^{0}+(\frac{1}{2}(0)^{2}e^{0}+(0)e^{0})-((0)e^{0}+e^{0})$ $c_{2}=2$ Substitute $c_{2}=2$ in $c_{1}+c_{2}=2$ to get $c_{1}=0$ Solution to the initial value problem is $y=2e^{x}+\frac{1}{2}x^{2}e^{x}-xe^{x}$
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