# How to take the fourier transform of a function?

1. Sep 16, 2012

### XcKyle93

1. The problem statement, all variables and given/known data
Find the fourier transform of x(t) = e-t sin(t), t >=0.

We're barely 3 weeks into my signals course, and my professor has already introduced the fourier transform. I barely understand what it means, but I just want to get through this problem set.

2. Relevant equations
I honestly don't know. I know it's an improper integral with bounds -∞ and ∞, that is
∫x(t)e-j2∏tdt

3. The attempt at a solution
I get something VERY LONG which does not seem right.

Last edited: Sep 17, 2012
2. Sep 17, 2012

### rude man

How can sin(t) expressed in exponential notation?

BTW your integral is missing dt. It is dt, right?

3. Sep 17, 2012

### XcKyle93

Yes, that is correct.

I know that sin(t) = (1/2) * j * (e-t - et), so x(t) = (1/2)*j*(et(-j-1) - et(j-1)). But then I'd have something really painful to integrate, right?

4. Sep 17, 2012

### rude man

You're right, can't do it that way. Will come back to you in a short while.

5. Sep 17, 2012

### rude man

Looks like convolution in the frequency domain is the way to go. You remember the convolution theorem?

So you'll need F(ω) for f(t) = exp(-at) and f(t) = sin(ωt), both for t > 0 and both = 0 for t < 0. The first one is easy; let me know how you're managing with the second ... remember when you take F(ω) to integrate from 0 to ∞, not -∞ to +∞.

EDIT: never mind convolution. You can do the integral.

The integral is not as bad as you think. Key point is that lim t→∞ {e-(a + jb)t} = 0 providing a > 0 which it is in your case.

Last edited: Sep 17, 2012