Optimization - area of rectangle

[look416]

Homework Statement

If the perimeter of a rectangle is fixed in length, show that the area of the rectangle is greatest when it is square

Homework Equations

The Attempt at a Solution

if the perimeter is fixed in length, then 2x + 2y = c
then no idea to continue from there

 

[Dick]

Ok so far. Now you want to maximize x*y, right? Eliminate one of the variables using the perimeter condition and differentiate, etc.

 

[rock.freak667]

The Attempt at a Solution

if the perimeter is fixed in length, then 2x + 2y = c
then no idea to continue from there

So what is the area A in terms of x and c?