Mechanics maths question- centre of mass

[yorkshire]

Right, so I have a mechanics module tomorrow morning, and I'm almost there, the only thing stumping me is the centre of mass of a uniform semicircular lamina.
I have the formula for a sector of a circle, which is

2r sin(angle)
sin (angle)
(r is radius)

Now when it's a semicircle then the angle would be ∏ radians, but sin(∏) is 0 therefore this formula isn't going to work. Am I missing something here?

I did google this and the answer I saw is
4r
3p


Yet nowhere I saw this did they say what p was. Is it short for pi?

Thanks for any help, this is getting me stressed out!

 

[miles01110]

Yes, the "p" is pi. It's not a particularly hard derivation; use polar coordinates.