# Toronto Math Forum

## MAT244-2014F => MAT244 Math--Lectures => Topic started by: Chang Peng (Eddie) Liu on December 06, 2014, 01:34:45 PM

Title: Particular Solution Question
Post by: Chang Peng (Eddie) Liu on December 06, 2014, 01:34:45 PM
$$y^{(4)}âˆ’3yâ€³âˆ’4y=\sin(t)+8t.$$

apparently the particular solution is $Y = At\cos(t) + Btsin(t) + Ct + D$

I know why it's $At\cos(t) + Bt\sin(t)$ , but I have no idea where the $Ct + D$ is coming from. Can someone please explain this?
Title: Re: Particular Solution Question
Post by: Victor Ivrii on December 06, 2014, 05:46:51 PM
Observe that $r_{1,2}=\pm 2$, $r_{3,4}=\pm i$.

Because right-hand expression is $f_1+f_2$ with $f_1=\sin(t)$ and $f_2=8t$. Then we need $Y=Y_1+Y_2$ with  $Y_1= (A\cos(t)+B\sin (t))t$ and $Y_2= Ct+D$.
Title: Re: Particular Solution Question
Post by: Chang Peng (Eddie) Liu on December 06, 2014, 08:08:35 PM
That makes sense. Thank you professor.
Title: Re: Particular Solution Question
Post by: Sheng Zang on December 06, 2014, 09:55:50 PM
Prof, why you make the particular solution Y1=(Acos(t)+Bsin(t))t^2?
I think for this question, the particular solution multiplies by t is enough, i.e Y1= Atcos(t)+Btsin(t).
Thank you.
Title: Re: Particular Solution Question
Post by: Victor Ivrii on December 06, 2014, 11:52:00 PM
Yes, it was a misprint. Corrected