Let x, y, and z denote the side lengths of the box, with the bottom face having dimensions x-by-y and z is the height. Naturally this means each of x, y, and z must be greater than 0.
The box has a fixed volume of 252 cm³, so
xyz = 252
The surface area of the box is
2xy + 2xz + 2yz
and we're told that the material cost for each face is different. The total cost of the material needed to make the box is given by
C (x, y, z) = ($5/cm²) xy + ($2/cm²) xy + 2 ($3/cm²) (xz + yz)
or, omitting units and simplifying,
C (x, y, z) = 7xy + 6 (x + y) z
In the volume constraint, solve for any one variable; I'll do z.
z = 252/(xy)
Substitute this into the cost function:
C (x, y, 252/(xy)) = 7xy + 1512 (x + y)/(xy)
Since this is now a function of 2 variables, I'll rewrite this as
C* (x, y) = 7xy + 1512 (1/y + 1/x)
Compute the partial derivatives of C and find the critical points:
∂C/∂x = 7y - 1512/x² = 0 ⇒ x² y = 216
∂C/∂y = 7x - 1512/y² = 0 ⇒ x y² = 216
It follows that
x² y = x y² ⇒ x = y
Just like before, we can think about C* as yet another function but only of 1 variable,
C** (x) = 7x² + 3024/x
Now find the critical points of C** :
dC**/dx = 14x - 3024/x² = 0 ⇒ x = 6
All of this tells us that C (x, y, z) has a critical point when x = y = 6 and z = 252/6² = 7. So the box that costs the least has dimensions 6 × 6 × 7 cm, which gives the box a surface area of 240 cm² and a total cost of $756.
