Answer to Question #13357 in Calculus for Anandi
R = radius sphere
r = base radius cone
R + h = height cone
V = volume cone
V = (1/3)πr²(R + h)
By the Pythagorean Theorem:
r² = R² - h²
Plug into the formula for volume.
V = (1/3)π(R² - h²)(R + h) = (1/3)π(R³ + R²h - Rh² - h³)
Take the derivative and set equal to zero to find the critical points.
dV/dh = (1/3)π(R² - 2Rh - 3h²) = 0
R² - 2Rh - 3h² = 0
(R - 3h)(R + h) = 0
h = R/3, -R
But h must be positive so:
h = R/3
Calculate the second derivative to determine the nature of the critical points.
d²V/dh² = (π/3)(-2R - 6h) < 0
So this is a relative maximum which we wanted.
Solve for r².
r² = R² - h² = R² - (R/3)² = R²(1 - 1/9) = (8/9)R²
Calculate maximum volume.
V = (π/3)[(8/9)R²](R + R/3) = (8/27)πR³(4/3) = 32πR³/81
For R = 3 maximum volume is:
V = 32π(3³)/81 = 32π(27)/81 = 32π/3
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