# Integral help cos and exp

I'm sure this integral has been seen before. I'm trying to do a convolution for optics homework. No answer returns

```
sage: var('x1, x2, y1, y2, L, z, r')
sage: integral(cos(x1/L)*exp(-i*(k/z)*(x1*x2 + y1*y2)), x1, -L/2, L/2)
```

thanks for any help!