Question 762364
Use x ft for breadth.
Length = 800 - 2x
Area = Breadth * Length
Area = x(800 - 2x)
Area(x) = 800x - 2x^2
Differentiate
Area'(x)= 800 - 4x
Area'(x) = 0
          800 - 4x = 0
             4x = 800
              x = 200
Check gradient on either side of 200
So maximum occurs when x = 200
So breadth = 200ft
Length = 400ft
Maximum area = 800(200) - 2(800)^2
             = 80000ft^2
Hope this helps.
:-)