*** Everything below is broken. Kept just for posterity ***
Install this script into the footer in the Code Injection section
<script type="text/javascript" async
src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.4/latest.js?config=TeX-MML-AM_CHTML">
</script>
(that should autoupdate MathJax versions or if it all breaks and goes to hell, get the latest install instructions here)
If I’ve done this right, this little piece of LaTeX should give me the quadratic equation
$$ x=\frac{-b\pm\sqrt{b^2-4ac}}{2a} $$
$$x=\frac{-b\pm\sqrt{b^2-4ac}}{2a} $$
Et voila!