Kepler's 3rd law of planetary motion is
The square of the time period of a planet orbiting a sun is proportional to the cube of the semi major axis of the elliptical orbit.
$$(T_1)^2 = (A_1)^3,$$
But in many other sites it (the square of the time period) is said to be proportional to the mean radius of the elliptical orbit.
$$(T_1)^2 = (R_1)^3,$$
which one is correct? Or is there a relation between the mean radius and the semi major axis which cancel out because of proportionality?