Explanation:
Key 3D curved solid formulas: Cylinder volume = πr2h, curved surface area = 2πrh; Sphere volume = 43πr3, surface area = 4πr2; Cone volume = 13πr2h, curved surface area = πrl (where slant height l = √(r2 + h2)).
For a solid hemisphere or solid cone, remember to include the flat circular base (+πr2), so the total surface area of a solid hemisphere is 2πr2 + πr2 = 3πr2. When melting and recasting shapes, equate their exact volumes in terms of π so π cancels cleanly.
Example 1: Find the exact total surface area of a solid cone with base radius 5 cm and vertical height 12 cm.
Solution:
l = √(52 + 122) = 13 cm
CSA + Base = π(5)(13) + π(5)2 = 65π + 25π = 90π cm2
Example 2: A metal sphere of radius 6 cm is melted down to form a cylinder of radius 4 cm. Find the height h of the cylinder.
Solution:
Vsphere = 43π(6)3 = 288π; Vcyl = π(4)2h = 16πh
16πh = 288π ⇒ h = 288 ÷ 16 = 18 cm