Orbit Parameters and Energy

bon

1. The problem statement, all variables and given/known data

Ok so take r' to be r dot..

i have E = 1/2m (r' ^2 + h^2/r^2) - A/r = E

Energy equation for central force (inverse square)

and r = l/(1+ecostheta) where l is the latus rectum..

need to derive the expression linking E and e i.e. E= A^2/2mh^2 (e^2-1) but cant seem to get there...

any help would be great..
thanks

2. Relevant equations



3. The attempt at a solution

well for a start, when you differentiate r, the result includes a term theta dot -- where does that go?!