Answer:
31,250 ft²
Step-by-step explanation:
Let x represent the length of fence parallel to the river. Then the ends of the rectangle will have dimesion (500-x)/2, and the total area will be ...
... A = x(500-x)/2
This function describes a parabola with zeros at x=0 and x=500. The vertex (maximum) will be located halfway between these, at x=250.
The maximum size pen has area ...
... A = 250(500 -250)/2 = 31,250 . . . . sq ft
