Answer:
total cost = 8x^2 +17280/x
Explanation:
Let x represent the base length. Then the area of the base is x^2, and the height is h = 720/x^2.
The area of the four sides is ...
(4x)(h) = (4x)(720/x^2) = 2880/x
The cost of the base is ...
base cost = 8x^2
And the cost of the sides is ...
side cost = 6(2880)/x = 17280/x
The total cost of the box is ...
total cost = base cost + side cost
total cost = 8x^2 +17280/x
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Comment on the cost function
You will find this function has a minimum at x=∛1080 ≈ 10.260 in. The total cost is about $2526.35, and the box is 2/3 times as tall as wide. That aspect ratio makes any pair of opposite sides cost the same as the base, the generic solution to a cost optimization problem of this sort.