Explanation:
A frustum is the solid remaining when a smaller cone (or pyramid) is sliced off the top of a larger cone parallel to its base. Because the removed top cone and the original full cone are mathematically similar, the ratio of their heights equals the ratio of their radii: hH = rR.
Use similar triangles first if the height of the removed top cone is not given, then calculate: Volume of frustum = Volume of large cone − Volume of small cone = 13πR2H − 13πr2h.
Example 1: A cone of radius 9 cm and height 15 cm has a smaller cone of radius 3 cm and height 5 cm removed from its top. Find the exact volume of the frustum.
Solution:
Vlarge = 13π(9)2(15) = 405π; Vsmall = 13π(3)2(5) = 15π
Vfrustum = 405π − 15π = 390π cm3
Example 2: A frustum has top radius 4 cm, base radius 10 cm, and vertical height 9 cm. Find the volume of the frustum (3 s.f.).
Solution:
hh + 9 = 410 ⇒ 10h = 4h + 36 ⇒ h = 6 cm (H = 15 cm)
V = 13π(102×15 − 42×6) = 468π = 1470 cm3 (3 s.f.)