1. Relevant problem integrate from 0 to infinity of r^2exp^(-r/a0)dr 2. Relevant equations I'm also given; integral from 0 to infinity of x^nexp^-x dx = n! 3. The attempt at a solution I'm just wondering if I can split up the exponential to make it look like this form. Eg; integrate from 0 to infinity of (r^2e^(-r/a0)dr becomes; integrate from 0 to infinity of (r^2e^(-r)dr times integrate from 0 to infinity of (e^(1/a0)dr however I'm pretty sure when I split up the integral, the second term isn't correct. Can anyone help? I just don't want to integration by parts alot of times. As there's two other terms with higher powers of r to go through.