The first one. (a b and c)

This is the formula for the volume
Area of the circle*height=100
(pi*r^2)*h = 100

Solving for Height to get everything in terms of radius.
100/(pi*r^2) = h

Formula for Surface Area(material):
Circumference*height + 2(area of the circle)
h(2r*pi) + 2(pi*r^2)

Plug in your value of h from above.
(100/(pi*r^2))(2r*pi)) + 2(pi*r^2) = SA

Differentiate that. Solve for 0. Find the minimum. Then plug it in.

I think. Been a while and am doing it in my head.




edit: explanations