A Rectangular Box With Volume 320 Ft3 Is Built With

A rectangular box with volume 320 ft3 is built with a square base and top. The cost is $1.50 / ft2 for the bottom, $2.50 / ft2 for the sides, and $1 / ft2 for the top. Let x = the length of the base, in feet.
A rectangular box with volume 320 ft3 is built with

(a) Express the cost of the box as a function of x.
(b) Find the domain of the function.
(c) Graph the function with a graphing calculator.
(d) What dimensions minimize the cost of the box?

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