Speed of a planet in an elliptical orbit with semi major axis 'a' about the sun of mass M at a distance 'r' from the sun is

if major axis is ‘a’ then mean distance is also a and total energy is -GMm2aat a distance r if the velocity is v then kinetic energy + potential energy = --GmM2a12mv2- GMmr=-GMm2av2=2GMr-GM2a   v=GM2r-1a