Vis Viva Equation Formula

Equation for calculate vis viva equation is,

$$v^2=G\times M\times (\frac{2}{r}-\frac{1}{a})$$

where,
G = Newtonian constant of gravitation (6.67384e-11 m³ kg^-1 s^-2),
M = mass of the central body,
r = distance between the two bodies,
a = semi-major axis,
v = relative speed of the two bodies