MathJax should automatically parse any TeX-like expressions in your posts,
however it does not do it in real time (at the moment). It may be possible to enable this.
Thus, expressions like $E=mc^2$ will appear beautifully in the rendered posting.
There are issues with the Markdown Editor (which does produce syntax highlighting nicely)
that it also interprets some characters as formatting, which may be part of your formula.
It is possible to disable these, if they are reported as problems...
$$ A_{ij} x_j = b_i $$
from this:
$$ A_{ij} x_j = b_i $$
Test some common formulae:
$$\langle v_j,v_k \rangle=\int_0^1 v_j v_k\,dx$$
$$\langle v_j,v_k \rangle=\int_0^1 v_j v_k\,dx$$
