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

Considering a two-body problem, star-planet, prove that the angular momentum of the planet is given by:

[tex]

\begin{equation}

{\cal L}_{e} = {\mu}\sqrt{GMa(1-e^2)},

\end{equation}

[/tex]

[tex]$\mu$[/tex] is the reduced mass

M is the star mass

a and e are the semi-major axis and eccentricity of the planet

**3. The attempt at a solution**

I have no idea how to start the problem. Can someone give me a light? what consideration should I make?

Thanks in advance