Integrating Radial Probability Densitiesd Explanation

[Brianrofl]

Homework Statement

Image: http://puu.sh/ca93V/7eb9abf342.png

Homework Equations

Ok I know to use this guy http://puu.sh/ca95c/ad0155a4d6.png

Which then turns into this http://puu.sh/ca96W/68802e045c.png (except from .5a to 4a, not 0 to a)

The Attempt at a Solution

I get lost on trying to integrate this thing. All these problems involving radial probability densities seem to have weird integrals. I've tried letting x = r/a but that also just gets weird.

If I could get an explanation on this topic I'd really appreciate it.

 

[haruspex]

Are you saying you just need help integrating $r^4e^{-kr}.dr$? Please post your attempt, as per forum rules.