Web: Configure and source MathJax using their content distribution network. Place these two scripts in your tumblr header.

mathjax-tumblr

    <!-- MathJax configuration -->
    <script type="text/x-mathjax-config">
        MathJax.Hub.Config({
            TeX: { equationNumbers: {autoNumber: "AMS"} },
            tex2jax: {
              inlineMath: [ ['$','$'], ["\\(","\\)"] ],
              processEscapes: true
            }
        });
    </script>
    
    <!-- MathJax CDN -->
    <script type="text/javascript" src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>