# Homework Help: Laplace Transform solution help

1. Jun 19, 2009

### Melawrghk

1. The problem statement, all variables and given/known data
Find the solution of the given initial value problem:
y''+4y=upi(t)-u3pi(t) y(0)=7, y'(0)=5

3. The attempt at a solution
So I found the L{} of the above equation:
s2Y-s*f(0)-f'(0)+4Y = (e-pi*s)/s-(e-3pi*s)/s

Combining and substituting the numbers I get:
Y=$$\frac{e^{-pi*s}-e^{-3pi*s}}{s(s^{2}+4)}$$+$$\frac{6s+3}{s^2+4}$$

I know how to do the second term's inverse Laplace, but not the first. Here is what I tried:
I can see that I can't get rid of the exponentials in any way other than using the step function again. And the other denominator factor (s^2+4) can be potentially used to get sine. So that:
Y=$$\frac{1}{2}(\frac{e^{-pi*s}-e^{-3pi*s}}{s})$$$$\frac{2}{s^{2}+4}$$

This is where I don't know what to do. I can't separate them and I don't know of a way to do Laplace inverse of a product.

Any help would be really appreciated. Thanks

Last edited: Jun 19, 2009
2. Jun 19, 2009

### Vid

partial fractions

Also, the exponentials should be powers of s not t.

3. Jun 19, 2009

### Melawrghk

How do I do partial fractions with exponentials? Do I use like Ae^(-pi*s) instead of the usual A?

Edit: I fixed the powers

Last edited: Jun 19, 2009
4. Jun 19, 2009

### Vid

Nevermind partial fractions won't help here. You need to have proven a theorem about the laplace transform of the step function times another function.

5. Jun 19, 2009

### Melawrghk

Nah, I got it, you do need partial fractions. Thanks.