This site uses QuickLaTeX to produce LaTeX-synatax equations. To get equations inside a comment, just write `[latexpage]` (with the brackets) anywhere in your comment, and then include the usual LaTeX notation, i.e., `$f_\alpha(x^2)$` becomes .

You can also use `$$...$$`, `\[...\]`, `\(...\)`, `\equation`, `\align`, `\displaymath`, `\eqnarray`, `\multline`, `\flalign`, `\gather`, and `\alignat`. In fact, if you really want, you write TikZ commands directly in the post using `\begin{tikzpicture}...\end{tikzpicture}`. QuickLaTeX will render them into images.

For more details, look here.