It seems relatively simple but I still can't get it figure out.
Use lagrange multipliers to prove that the triangle with maximum area that has a given perimeter p is equilateral.
where and x, y, z are the lengths of the sides.
It's easier to maximise A^2 than A. So the problem is to maximise s(s–x)(s–y)(s–z) subject to the constraint x+y+z=2s, where s is constant. (There's also the condition that all the quantities x, y, z, s–x, s–y, s–z should be positive.) That should be a pretty standard Lagrange multiplier problem.