Latex doesn't work and you want to write a formula ?
Go to this site : LaTeX
it's in french, but all you gotta do is to type the formula (without the math tags (Rofl)) and click on "Afficher", which means "display"
Be careful to copy the code if you need it for later because most of the time you can't get it back.
After that, you'll get a picture and 2 links (in bold) below.
The first link is the link of the image, and the second one is the bbcode. (I'm saying this because it's in French, not because I think you're dumb ! xD)
P.S. : note that some of the common code you know here won't work, for example "\tfrac" "\implies" ... :)