Sticking this after my initialization #+ stuff finally worked:

#+begin_html
<script type="text/x-mathjax-config">
 MathJax.Hub.Config({
   TeX: {extensions: ["cancel.js"]},
   tex2jax: {
     inlineMath: [ ['$','$'], ['\\(','\\)'] ],
     processEscapes: true
   }
 });
</script>
#+end_html

although I, the eternal beginner, don't quite understand other than MathJax
is pulling in this cancel.js and applying it. Got it off the MathJax Google
Group. I'm sure there's a better way to roll it into Emacs/org-mode
or org-html-mathjax-template?