Question 1138949
Draw this
sides are x x and 400-2x
The area is x(400-2x)=-2x^2+400x
the vertex for this quadratic is a maximum and will have x value of -b/2a=-400/-4=100 yds
the maximum area for this 100*200 rectangle is 20000 yds^2.

The formula for A=-2W2+400W for W=width