Global Maximum for area of a triangle.

Hello,

I need to find the length of the base of a (isoceles) triangle (x) to get the maximum area in terms of the fixed sides (L)

so far I have found the critical points of the derivative

Area = (1/2) xh

Where h = ( L^2 - (x^2)/4)^1/2

L = Fixed lengths of triangle = constant

So when derivative = 0, we have

Area' = x = L* (2)^1/2

I just need to know how i can test if this is a global maximum of the fucntion.

do i just find the limits when x goes to 0 and infinity? What does it mean when i do?

Cheers