A closed box with a square base is to have a volume of 13 comma 500 cm cubed. The material for the top and bottom of the box costs $10.00 per square centimeter, while the material for the sides costs $2.50 per square centimeter. Find the dimensions of the box that will lead to the minimum total cost. What is the minimum total cost?

x  =  1,5 cm

h  =  6  cm

C(min) = 135 $

Step-by-step explanation:

Volume of the box is :

V(b)  = 13,5 cm³

Aea of the top is equal to area of the base,

Let call  " x " side of the base then as it is square area is A₁ = x²

Sides areas are 4 each one equal to x * h  (where h is the high of the box)

And volume of the box is   13,5 cm³  = x²*h

Then   h  =  13,5/x²

Side area is :  A₂ =  x* 13,5/x²

A(b)  = A₁  + A₂

Total area of the box as functon of x is:

A(x)  = 2*x²  + 4* 13,5 / x

And finally cost of the box is

C(x)  = 10*2*x²   +  2.50*4*13.5/x

C(x)  = 20*x²  +  135/x

Taking derivatives on both sides of the equation:

C´(x)  =  40*x   -  135*/x²

C´(x)  = 0     ⇒      40*x   -  135*/x² = 0    ⇒  40*x³ = 135

x³  = 3.375

x  = 1,5 cm

And   h  =  13,5/x²     ⇒   h  =  13,5/ (1,5)²

h = 6 cm

C(min)  = 20*x²  + 135/x

C(min)  = 45  +  90

C(min)  = 135 $

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