I hope you can help me :)

[Luz]

I have this problem from my pre-calculus class, but I really don't get it. I guess I should use the first derivative criteria, but I don't know how I can get there.
Please
A large conical tank will be manufactured with a circular metal piece with a radius of 10 meters, cutting a sector with an angle u and then welding the straight sides of the remaining piece. Find u so that the resulting cone has the largest possible volume.

I know the volume of the cone is V= 1/3 pi r^2 (h) and the last steps I should do, the problem is I don't have any idea about where to start.

 

[skeeter]

for the cone formed ...

slant height = radius of the circular sheet = [math]R = 10 m[/math]circumference of the cone’s circular base = [MATH]2\pi R - Ru = R(2\pi - u)[/MATH], where [MATH]u[/MATH] is in radians.
radius of the cone’s circular base, [MATH]r = \dfrac{R(2\pi - u)}{2\pi}[/MATH]
cone height squared = cone slant height squared - cone base radius squared

see what you can do from here ...