Question

A rectangular pen is made with 100 m of fencing in three sides. The fourth is a stone wall. Find the greatest possible area of such an enclosure.

Answer 1

First draw it out and write your variables out. You know that the perimeter (2y +x) must be 100. You also know the area equation. Get y in terms of x and replace it into the area function. Then you can take the derivative of that and set it to 0.