The radius of the completed cone is
... 18×(300°/360°) = 15
The height of the completed cone is found from the Pythagorean theorem:
... 15² + h² = 18²
... h = √99 = 3√11
The volume is given by
... V = (1/3)πr²·h = (1/3)π·15²·3√11
Dividing this value by π gives 225√11 cubic units.
