### Author Topic: FE-3  (Read 4126 times)

#### Victor Ivrii

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##### FE-3
« on: April 17, 2013, 03:02:14 PM »
\begin{equation*}
y^{(4)} -3y''-4y = \sin (t)+8 t.
\end{equation*}

#### Jason Hamilton

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##### Re: FE-3
« Reply #1 on: April 17, 2013, 04:28:47 PM »
solve homogeneous->  roots are i,-i,2,-2

Yh=c1 e^2t +c2 e^-2t +c3 cost +c4 sint

Yp-> undetermined coefficients-> Yp= Atcost +Btsint +Ct +D

evaulate polynomial terms => C=-2. D=0

evaulate sinusoidal terms => A=1/10 B=0

Y= c1 e^2t +c2 e^-2t +c3 cost +c4 sint + (1/10)tcost -2t

#### Jason Hamilton

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##### Re: FE-3
« Reply #2 on: April 17, 2013, 04:31:38 PM »
Also, I MIGHT have used 'x' instead of 't' on the test, would I lose marks if I used the wrong variable?

#### Victor Lam

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##### Re: FE-3
« Reply #3 on: April 17, 2013, 06:14:16 PM »
i did this

#### Benny Ho

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##### Q3
« Reply #4 on: April 17, 2013, 09:17:02 PM »
Solutions

#### Victor Ivrii

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##### Re: FE-3
« Reply #5 on: April 17, 2013, 11:25:23 PM »
Benny, WTH you posted Q1 into FE-3 and Q3 into FE-1 (so I needed to split and merge)?