### Author Topic: Day Section Problem 1  (Read 4295 times)

#### Victor Ivrii

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##### Day Section Problem 1
« on: February 27, 2013, 07:46:14 PM »
Find the general solution of
\begin{equation*}
y'''-y''+y'-y=e^{-t}\sin(t).
\end{equation*}
« Last Edit: February 28, 2013, 11:01:34 AM by Victor Ivrii »

#### Changyu Li

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##### Re: Day Section Problem 1
« Reply #1 on: February 28, 2013, 11:29:34 AM »
$$(r-1)(r^2+1) = 0 \\ r = 1, \pm i \\ y_h = c_1 e^t + c_2 e^{it} + c_3 e^{-it} \\ y_p = A e^{-t} \sin t + B e^{-t} \cos t \\ y_p' = e^{-t} \left(\left(A-B\right) \cos t - \left(A+B\right) \sin t \right) \\ y_p'' = -2 e^{-t} \left(A \cos t - B \sin t\right) \\ y_p''' = 2 e^{-t}\left(\left(A-B\right) \sin t + \left(A+B\right) \cos t \right) \\ A = 0, B = -\frac{1}{5} \\ y = c_1 e^t + c_2 e^{it} + c_3 e^{-it} -\frac{1}{5}e^{-t} \cos t$$
« Last Edit: February 28, 2013, 11:46:14 AM by Victor Ivrii »

#### Victor Ivrii

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##### Re: Day Section Problem 1
« Reply #2 on: February 28, 2013, 11:48:52 AM »
Please do not increase font size; also in this and another Quiz 3 problem provide solution in the real form as combining complex exponents and $\sin$. $\cos$ creates an eclectic mess