\magnification=\magstep2

$$
   P^2  =  { {4 \pi^2} \over {G (M_s + M_p)} }  (R_s + R_p)^3
$$


$$
   P^2  \sim  { {4 \pi^2} \over {G (M_s)} }  (R_p)^3
$$


\bye
