Author Topic: Problem 2, night sections  (Read 2300 times)

Victor Ivrii

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Problem 2, night sections
« on: October 30, 2013, 08:11:06 PM »
Find the general solution of the given differential equation:
\begin{equation*}
y''-y'-2y = -2t + 4t^2.
\end{equation*}

Yangming Cai

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Re: Problem 2, night sections
« Reply #1 on: October 30, 2013, 08:51:01 PM »
answers as follow

Ka Hou Cheok

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Re: Problem 2, night sections
« Reply #2 on: October 30, 2013, 08:54:48 PM »
Let the solution $y=y_c+Y$,

The characteristic equation for the homogeneous equation $y''-y'-2y=0$ is
$$r^2-r-2=0$$
Solving the equation we have $r_1=2, r_2=-1$ and hence $$y_c=C_1\exp(2t)+C_2\exp(-t)$$

Let $Y=At^2+Bt+C$, $Y'=2tA+B$, $Y''=2A$.

$Y''-Y'-2Y=(2A)-(2tA+B)-2(At^2+Bt+C)=(-2A)t^2+(-2A-2B)t+(2A-B-2C)=-2t+4t^2$

By comparing the coefficients,
$$
\left\{\begin{aligned}
&-2A=4,\\
&-2A-2B=-2,\\
&2A-B-2C=0.
\end{aligned}\right.
$$
Then,
$$
\left\{\begin{aligned}
&A=-2,\\
&B=3,\\
&C=-7/2.
\end{aligned}\right.
$$
So, $Y=-2t^2+3t-\frac{7}{2}$ and hence $$y=y_c+Y=C_1\exp(2t)+C_2\exp(-t)-2t^2+3t-\frac{7}{2}$$
« Last Edit: October 31, 2013, 05:35:19 AM by Victor Ivrii »

Tianqi Chen

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Re: Problem 2, night sections
« Reply #3 on: November 01, 2013, 11:24:06 AM »
Question2