MathJax is a display engine for complex mathematic formulas. Notebooks supports MathJax in combination with Markdown. To activate MathJax support for your Markdown documents,

Alternatively you can add the following lines at the bottom of your favorite document style sheet (right after the </style> tag) to turn on MathJax support for that style.

You can even add these lines at the top of a Markdown document to enable MathJax for that document.

<script type="text/javascript"
src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
</script>

<script type="text/x-mathjax-config">
MathJax.Hub.Config({
tex2jax: {
inlineMath: [ ['$','$'], ["\$$","\$$"] ],
displayMath: [ ['$$','$$'], ["$","$"] ]
}
});
</script>

Use Mathematic Expressions in Documents (MathJax)