# Toronto Math Forum

## MAT244-2013S => MAT244 Math--Tests => Quiz 3 => Topic started by: Victor Ivrii on February 27, 2013, 07:46:14 PM

Title: Day Section Problem 1
Post by: Victor Ivrii 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*}
Title: Re: Day Section Problem 1
Post by: Changyu Li on February 28, 2013, 11:29:34 AM
$$(r-1)(r^2+1) = 0 \\ r = 1, \pm i \\ y_h = c_1 e^t + c_2 e^{it} + c_3 e^{-it} \\ y_p = A e^{-t} \sin t + B e^{-t} \cos t \\ y_p' = e^{-t} \left(\left(A-B\right) \cos t - \left(A+B\right) \sin t \right) \\ y_p'' = -2 e^{-t} \left(A \cos t - B \sin t\right) \\ y_p''' = 2 e^{-t}\left(\left(A-B\right) \sin t + \left(A+B\right) \cos t \right) \\ A = 0, B = -\frac{1}{5} \\ y = c_1 e^t + c_2 e^{it} + c_3 e^{-it} -\frac{1}{5}e^{-t} \cos t$$
Title: Re: Day Section Problem 1
Post by: Victor Ivrii 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