Bradford T. answered 03/18/21
Retired Engineer / Upper level math instructor
Volume of a cylinder V = πr2h where r≡radius and h≡height
h = V/(πr2) = 600/(πr2)
Surface area = 2πr2 + 2πrh (Twice top and bottom and the side surface areas)
Cost, C = 0.06(2)πr2 + 0.02(2πrh)
Substituting for h
C(r) = 0.12πr2 + 0.04πr(600/πr2) = 0.12πr2 + 24/r
To minimize the cost, take the derivative of the cost, set that to zero and solve for r.
C'(r) = 0.24πr - 24/r2
0.24πr = 24/r2
r3 = (24/0.24)/π = 100/π
r = (100/π)1/3 = 3.16 cm
h = 600/(π(3.16)2) = 19 cm
C = 0.12πr2 + 24/r = 3.76 + 7.59 = 11.35 cents