Nikola + Pandoc + Mathjax: Math not displayed properly

Question

I have a static site with Nikola in which .md are turned into .html with pandoc.

Part of my conf.py looks like:

COMPILERS = {
    # "markdown": ('.md', '.mdown', '.markdown'),
    "pandoc": ('.md', '.mdown', '.markdown')
}

I generate posts using nikola new_post -f pandoc.

My question is about generating Maths. I have a .md file like this:

<!--
.. title: Testing Maths
.. slug: testing-maths
.. date: 2015-07-04 23:44:07 UTC+08:00
.. tags: 
.. category: 
.. link: 
.. description: 
.. type: text
-->

$\beta_i=2$
$k_i=2$
$\hat{\beta_i}=2$

I build my site with nikola build and host it locally with nikola serve. And here's a screenshot of the browser:

result http://s9.postimg.org/70t4j7k6l/Screen_Shot_2015_07_04_at_11_48_40_pm.png

Can anyone help? Thanks.