# Homework Help: How to find La Place transform of cos(x) * unit step function (x - pi)

1. Apr 12, 2013

### lonewolf219

1. The problem statement, all variables and given/known data

Find the La Place transform of cos(x)*(u(x-$\pi$))

2. Relevant equations

L{u(t-a)}(s)=(e^(-as))/s

3. The attempt at a solution

I don't think I can just multiply this by the La Place transform of cos (x), which is s/(s^2) ?

2. Apr 12, 2013

### lonewolf219

I apologize if I am posting in the wrong forum..

3. Apr 12, 2013

### MisterX

You're right, you wouldn't be able to just multiply the Laplace transforms together. You can write out the integral, and use the unit step to change the limits of integration. Then, you can solve it by integration by parts, I think.

4. Apr 12, 2013

### LCKurtz

You might use$$\mathcal L(f(t)u(t-a)) = e^{-as}\mathcal L(f(t+a))$$

5. Apr 12, 2013

### lonewolf219

Thanks for both suggestions!!! I appreciate the help