Suppose you wish to enclose a field for corn corn by a fence the is 2x feet long and y feet wide.  The perimeter of the fence is 800 feet.  What is x and y and what dimensions will maximize the area of the fence?

Perimeter: 2x + 2x + y + y = 800

                4x + 2y = 800

Area = 2x(y)

Solve for y in the equation for perimeter and substitute in the equation for area

2y = 800 - 4x

y = 400 - 2x

Area = 2x(400 - 2x)

        = 800x - 4x^2


To maximize the area, take the derivative of the area and set equal to 0 and solve for x.

Derivative of Area = 800 - 8x

Therefore, x = 100

y = 400 - 2(100)

  = 200

The dimensions of the field which gives maximum area is 200 x 200 (which makes sense, a square will always maximize area)