Volume of hemisphere of radius 9 cm

= × π × 9 × 9 × 9cm³

Volume of circular cone (height = 72 cm)

× π × r² × 72cm

Volume of cone = volume of hemisphere

× π × r² × 72 = π × 9 × 9 × 9

r² = π × 9 × 9 × 9 × π = 20.25

r = = 4.5cm

Hence, radius of the base of the cone = 4.5 cm