How do I express this problem?

Question

A closed box with a square base is required to have a volume of 50 cubic feet. Express the amount A of material used to make such a box as a function of the length x of a side of the square base.

Answer 1

Let h is the height of the box.
h x² = 50
h = 50/x²
Material needed (in square feet) is
A = 2 x² + 4 x h = 2 x² + 4 x 50/x² = 2 x² + 200/x
Can you find how to make the box with minimum amount of material?
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