$\frac{4}{3}\pi (\frac{RH}{R+\sqrt{R^2+H^2}})^3$

$\frac{4}{3}\pi (\frac{R+H}{2})^3$

$\frac{4}{3}\pi (\frac{RH}{R+H})^3$

$\frac{4}{3}\pi (\frac{R\sqrt{R^2+H^2}}{H})^3$