Illustrate the relation g ∝ 1/R² with an example.

g is inversely proportional to R²

From Newton's universal law of gravitation we have,
F = GMm/ R²      .............(i)
Force experienced by a body of mass m due to the gravitational field of mass M alone is given by,
F = mg              .................(ii)
From ....(i) and ....(ii) 
GMm/ R² = mg 
g = GM/ R²
As G and M are constant, it is proved that g is inversely proportional to 1/R².
g ∝ 1/R²