# Weird Exponential Integral

1. Nov 2, 2006

### steelphantom

Ok, this isn't a particularly hard integral, but for some reason I don't understand why the answer is what it is. Here's the integral (BTW, it's part of a double integral):

$$\int e^{x/y} dx$$

The answer is: $$ye^{x/y} dy$$ but I don't understand why.

Wouldn't it be in the form e^u, with u being x/y, and du being 1/y dy? If so, then the answer should be $$e^{x/y}/y dy$$, right? That's wrong I guess, because the rest of the integral is pretty much impossible to do if that's the answer. So basically, my question is, why is the answer the answer? Thanks!

Last edited: Nov 2, 2006
2. Nov 2, 2006

### Galileo

You're integrating wrt x, so just consider y constant.
The derivative of yexp(x/y) wrt x is exp(x/y) by the chain rule, so that's the correct answer.

If you use substitution u=x/y, then you should replace dx with ydu (not use dy, since you're integrating wrt x).

3. Nov 2, 2006

### steelphantom

Ah! That makes sense, especially since there's already a dy in the problem. No use having two of them. Thanks for the help!