differentiation

As the height is a constant, asuume H. Let the height and the radius of the cylinder be h, r respectively.
The curved surface area (S) of the cylinder is :
2πrh
= 2πr(16 - r)*(H/16) . . . . (Using the ratio of similar △)
= (πH/8)(16r - r^2)
As S = (πH/8)(16r - r^2)
dS/dr = (πH/4)(8 - r)
When dS/dR = 0, r = 8
So, when the radius of the cylinder is 8cm, its curved surface area is the greatest.

What is the height of the cone ? 10 cm.? 100cm.? 10000 cm.?......