Answer:
For the rectangular area, we will have a length L, and a width W, such that the two sides with measure L and one with measure W will be made with the material of $4 per ft, and the other side with measure W will be made with the material that costs $13 per ft.
Then the total cost will be:
C = (L + L + W)*$4 + W*$13.
And we know that the area is equal to 240 ft^2, then:
240ft^2 = L*W
To solve this, the first step is to isolate one of the variables in the second equation. I will isolate L:
L = (240ft^2)/W
Now we can replace this in the first equation to get:
C = ( 2*(240ft^2)/W + W)*$4 + W*$13.
C = $4*480ft^2/W + W*$17
Now we want to find the minimum of this function, then we need to look at the zero of the first derivative of C:
C' = - ($4*480 ft^2)/W + W*$17 = 0
- ($4*480 ft^2) + $17*W^2 = 0
Let's solve this for W:
$17*W^2 = ($4*480 ft^2)
W^2 = ($4*480 ft^2)/$17 = 112.94ft^2
W = √(112.94 ft^2) = 10.63 ft.
Then the width must be 10.64 ft, and the length can be obtained with the equation:
L*W = 240 ft^2
L*10.64 ft = 240 ft^2
L = (240 ft^2)/10.64ft = 22.56 ft
The width is 10.64 ft, and the length is 22.56 ft
