Draw a picture, make x be the width of the farmland and y be the length.
You should find that 2x+y=800 so y = 800-2x
A = xy = x(800-2x)
Now solve A'=0
A rectangular plot of farmland will be bounded on one side by a river and on the other 3 sides by a single-strand electric fence. With 800m of wire at your disposal, what is the largest area you can enclose, and what are its dimensions?
Another problem I have no idea how to do. Any help is appreciated, thanks.