State Kepler's third law and derive mathematically to obtain constant.

Explanation

Kepler's third law: The square of its period of revolution around the Sun is directly proportional to the cube of the mean distance of a planet from the Sun.
It T is a periodic time and the average distance of a planet from the Sun is `r` then
`T^(2)alphar^(3)`
`:.T^(2)=kr^(3)`.....k is constnat `k!=0`
`:.k=(T^(2))/(r^(3))`