# Homework Help: Integrating exponentials

1. Jan 24, 2007

### Brewer

1. The problem statement, all variables and given/known data
I'm being dead thick, but I can't remember how to integrate an exponential function.
$$\int x^3e^{-\alpha x^2}dx$$

2. Relevant equations

3. The attempt at a solution
I reckon that this shouldn't be too complex, but I've totally forgotten how to go about this question. The $$x^3$$ term means that it can't be integrated like I would normally do (i.e. $$\int ke^x = ke^{\frac{x}{k}}$$) can it? I've also tried substituting $$u=x^2$$ into it and following that through, but it doesn't seem to get anywhere.

Is this really simple, and my recent lack of practice with the techniques is just failing me, or there something more complicated about it?

Last edited: Jan 24, 2007
2. Jan 24, 2007

### AlephZero

Substituting u = x^2 is a good first move.

You will get a simpler integral involving e^{-au} which you can solve by integration by parts.

3. Jan 24, 2007

### HallsofIvy

$$\int x^3e^{-\alpha x^2}dx= \int x^2 e^{-\alpha x^2} (xdx)$$

Now, as others have said, let u= x2.

4. Jan 25, 2007

### Brewer

Oh so I was going right.

I hate integration by parts.