Minumum surface for a given volume

[look416]

Homework Statement

An open tank is constructed with a square base and vertical sides to hold 32 cubic metres of water. Find the dimensions of the tank if the area of sheet metal used to make it is to have a minimum value

Homework Equations

The Attempt at a Solution

i just know i need to find the second derivatives of it, but don't know the purpose of it

 

[rock.freak667]

If the base has dimension x and the height is h, what is the volume of the tank? What is the area of the tank, A , equal to in terms of x and h?