Exercise given in textbook:
A farmer has 40m of fencing with which to enclose a rectangular pen.Given that the pen is x m wide - show that it's area is (20x - x^2)m^2 and deduce that the maximum area that he can enclose (Answer given 100m^2).
I have completed lots and lots of exercises involving completing the square- also with minimum and maximum values but am obviously missing something here.