In central force motion, the angular momentum of body remains conserved.
i.e., Angular momentum, L=r×p=mvr
[at minor and major axis v and r are perpendicular]
So, if v1 and v2 be the speed of earth at A and B, respectively
Then, mv1r1=mv2r2 ⇒v2v1=r1r2=OAOB ⇒v2v1=R