The complete question is
A fence must be built to enclose a rectangular area of 5000ft^2. Fencing material costs $1 per foot for the two sides facing north and south and 2$ per foot for the other two sides. Find the cost of the least expensive fence.
Answer:
Total cost will be $400
Explanation:
Let x = be other side
y = north and south side
Area = x*y = 5000
Perimeter of the rectangle = 2x + 2y
cost of fencing = 2(1)*5000/x + 2*2x
= 10000/x + 4x
now to get the least we will take the derivative of this
C'(x) = 10000(-1/x^2) + 4 =0
x^2 = 2500
x= 50ft cost = 2*$2*50 = $200
y= 100ft cost = 2*$1*100 = $200
Total cost = $400