Ooh... I should have guessed earlier that you are in the 'Go Skin'. There are several interfaces of the site, the most recent one must be the one you are using and I must tell you that I don't like it that much... instead of inserting the [code], replace 'code' by 'math' and insert those between the tabs.
cosA+cosB+cosC+1=4cos\(\(\frac{A}{2}\) cos\(\(\frac{B}{2}\) cos\(\(\frac{C}{2}\)
to give:
There is some error in the LaTeX again I don't know why... there normally should be a single "\(" but it doesn't appear when I put a single bracket... well...