1st step: declare the cost function:
cost = area_found*rate_found + surface_side*rate_side+surface_proof*rate_proof.
2nd step: let r and h the radius and the height of the semi-cylinder.Express the area_found and the 2 kind of surface with r and h
3th step: Look for a connection between h and r. We have to remark that r and h is not independent because volumen V is fix --> V(r, h) = const.
4th step: express the cost function with only one variant (r or h). So we get cost(r) function.
5th step: look for the minimum