Awkward integration

[Piano man]

Hi,
I'm reading up on Kepler's problem at the moment, and there's a step in the book that I don't understand.

Starting off with the equation of the path \phi=\int\frac{M dr/r^2}{\sqrt{2m[E-U(r)]-M^2/r^2}}+\mbox{constant}

The step involves subbing in U=-\alpha/r, 'and effecting elementary integration' to get
\phi=\cos^{-1}\frac{(M/r)-(m\alpha/M)}{\sqrt{(2mE+\frac{m^2\alpha^2}{M^2}})}+\mbox{co nstant}

But it doesn't look very elementary to me:confused:

Does anyone have any idea?

Thanks.

[vela]

Try the substitution w=1/r and then complete the square in the radical.