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## Homework Statement

By considering the centripetal force which acts on a planet in a circular orbit, show that T^2 is proportional to R~^3 where T is the time taken for one orbit around the sun and R is the radius of the orbit

## Homework Equations

I'm thinking keplers third law?

## The Attempt at a Solution

C=2piR

P=2piR/ sqrt*GMm/r

rearranged to give P = 2pi sqrt*R^3/GM

Lost now..