以下の記述をheader.phpに挿入する

 <script type="text/x-mathjax-config">
 MathJax.Hub.Config({
   tex2jax: {
     inlineMath: [['$','$'], ['\\(','\\)']],
     processEscapes: true
   }
 });
 </script >
 <script async src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js?config=TeX-AMS_CHTML"> </script>