Let 
and 
be the sides of the rectangle. The perimeter is given to be 500m, so we are maximizing the area function 
subject to the constraint 
.
From the constraint, we find
so we can write the area function independently of:
Differentiating and setting equal to zero, we find one critical point:
which means
, so in fact the largest area is achieved with a square fence that surrounds an area of 
.
From the constraint, we find
so we can write the area function independently of
Differentiating and setting equal to zero, we find one critical point:
which means