A farmer has 1800 feet of fence and wants to enclose a rectangular field with two internal fences parallel to one side, as shown below

A-area W*L W=A/L fencing 4W+2L=1800 4A/L + 2L=1800 mulptiply by L 4A+2L^2 = 1800L dA/dL=o for max/min dA/dL=4L-1800=0 for max L=1800/4=450 2L=900 4w=1800-900=900 w=900/4=225 so L=450 W=225