Ta có: \({V_} = \frac{1}{3}\pi {R^2} \cdot SO,\,\,{V_} = \frac{1}{3}\pi {R^2} \cdot SO'\)

Mặt khác, \(\Delta SO'A\) và \(\Delta SOB\) đồng dạng nên \(\frac{{R'}}{R} = \frac{{SO'}}\).

Suy ra: \(\frac{}{} = \frac{{{{R'}^2} \cdot SO'}}{{{R^2} \cdot SO}} = {\left( {\frac{{SO'}}} \right)^3} = \frac{1}{8}\).

Do đó \(\frac{{SO'}} = \frac{1}{2} \Rightarrow SO' = \frac{1}{2} \cdot 40 = 20\;\,({\rm{cm)}}.\)