Question

A rectangular area of 3200 ft 2 is to be fenced off. Two opposite sides will use fencing costing $1 per foot and the remaining sides will use fencing costing $2 per foot. Find the dimensions of the rectangle of least cost.

Answer 1

If the sides of the rectangle are a and b,
Area=ab=3200^2
If the side with a ft cost $2/ft and side with bft costs $2/ft
Then,
Cost = 2*2*a+2*1*b=4a+2b
At minimum cost (or critical point), the derivative of cost =0
From area, b=3200/a
Using cost equation
C=4a+2*3200/a=4a+6400/a
First derivative with respect to a;
C'(a)=4-6400/a^2=0
Then, a=+/-40 The negative value is rejected. Therefore, a=40 ft and b=3200/40= 80 ft.
Therefore, a=40 ft and b=80 ft to minimize the fencing cost.