Posted by Catie on .
A farmer wants to build a rectangular pen and then divide it with two interior fences. The total area is to be 2484ft^2. The exterior fence costs $18.00 per foot and the interior fence costs $16.50 per foot. Find the dimensions of the pen that will minimize the cost. What is the minimum cost?

Calculus 
Steve,
if width=x, length=y, the cost=c, we have
xy = 2484
c = 16.50*2x + 18.00*(2x+2y)
= 33x + 36x + 36(2484/x)
= 69x + 89424/x
dc/dx = 69  89424/x^2
dc/dx=0 when x=36
so, the cost is minimum when the pen is 36 by 69.
I assume you can figure the cost.