Derive angular momentum of planet with elliptical orbit

[psychicist]

Homework Statement

A planet of mass m orbiting the Sun(mass=m_s along an elliptical orbit, with aphelion r1 and perihelion r2. Find the angular momentum of the planet relative to the centre of the Sun.

Homework Equations

Angular momentum, M=m(R\times v)=mvR sin\theta
Distance from Sun to the planet, R=\frac{2{r_1}{r_2}}{(r_1+r_2)-(r_1-r_2)cos \theta}
while \theta is the angle of \overrightangle{R} from the semi-major axis, which varies with time.

The Attempt at a Solution

Known that the angular momtentum is conserved along its motion, with v, R, and \theta varies with time but not sure that whether it is useful to derive the value of R, and I don't know how to relate this 3 variables together. Can anyone help me? Thank you very much!

 

[Irid]

Oh just use energy conservation to equate energies at perihelion and aphelion and you'll be able to get a velocity at either point. Plug it into the corresponding angular momentum formula!