A storage shed is to be built in the shape of a box with a square base. It is to have a volume of 343 cubic feet. The concrete for the base costs ​$2 per square​ foot, the material for the roof costs ​$3 per square​ foot, and the material for the sides costs ​$2.50 per square foot. Find the dimensions of the most economical shed.

Respuesta :

Answer:

x = 7 ft

h = 7 ft

Step-by-step explanation:

Let call  "x " side of the square base and " h " the height of the shed, then

V(s) =  x² * h     ⇒   343  =  x² * h    ⇒  h  =  343 / x²

Total cost of the shed is

C(t)  =  Cost of the base (C₁)  +  Cost of the roof  (C₂ ) + Cost of 4 sides (C₃)

Then

C₁  = 2*x²             C₂  =  3*x²          and     C₃  =  4*2,50*x*h

In C₃  as  h  =  343 / x²       Then    C₃  =  10 *343/x     C₃ = 3430 /x

Total cost of the shed  as a function of x is:

C(x) = 2*x²   +  3*x²   +  3430 / x      (1)

Taking derivatives on both sides of the equation we get:

C´(x)  =  10*x  -  3430 / x²

C´(x)  =  0   ⇒   10*x -  3430 / x²  = 0

x  - 343 /x²   =  0     ⇒  x³  = 343

x  =∛ 343

x = 7 ft

If we plug this value in equation 1 we see C(x) > 0 then for x = 7 , we get a minimun in the function

h = 343 /x²

h = 7 ft

The computation of the equation shows that the dimensions will be 7 feet.

How to solve the equation?

The equation can be illustrated below:

Volume = x²y = 343

Price = 2x² + 3x² + (2.50)(4xy) = 5x² + 10xy.

Therefore, 5x² + 10x(353/x²) = 5x² + 3430/x

Taking the first order derivative, x² = 343

Therefore, x = 7

In conclusion, the dimensions are 7ft.

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