Question

Find the dimensions of an open rectangular glass tank of volume 4 cubic feet for which the amount of material needed to construct the tank is minimize

Answer 1

The base will have the greatest area for a given perimeter if it is square. If the edge of the square base has length x (in feet), then the total material requirement in square feet is
.. m = x^2 +(4/x^2)*(4x)
.. m = x^2 +16/x
This will have a minimum where dm/dx = 0.
.. dm/dx = 2x -16/x^2 = 0
.. x^3 = 8 . . . . . . . . . . . . . . . multiply by x^2/2 and add 8
.. x = 2

The tank is 2 feet square and 1 ft high.

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You will note that it is half the height of a cube that has double the volume. This is the generic solution to all minimum cost open-top box problems. Actually, the costs of pairs of opposite sides are equal to each other and to the cost of the base. If material costs are not identical in all directions, that is the more generic solution.