Cylinder radius and height

A closed cylinder is required to have a volume of 40m^3 but made with the minimum amount of material. Determine the radius and height the cylinder must have to meet such a requirement.


V= πr^2h


Steps needed:


a) Insert value and transpose for h
b) Then sub into the area formula
c) Then differentiate

(i) Write down Surface Area in terms of r and h.
(ii) Equate volume formula to 40
(iii) Express h in terms of r from Volume formula and hence get an expression for the surface area in terms of r only.
(iv) Differentiate with respect to r and use usual method to find value of r that makes A a minimum.

I have differentiated and got 4πr-80r^-2 is that correct?

What do you mean use usual method to find value of r that makes A a minimum I don't understand? Is there an equation needed?