Enclosing the Largest Area

The owner of the Rancho Los Feliz has 3420 yd of fencing to enclose a rectangular piece of grazing land along the straight portion of a river and then subdivide it by means of a fence running parallel to the sides. No fencing is required along the river. (See the figure below.) What are the dimensions of the largest area that can be enclosed?

x=________

y= _______

What is this area?

________ yd²