mathml/test_tex.tex

\documentclass{article}
\usepackage{unicode-math}
\directlua{require'test_tex'}
\begin{document}
\[
  ax^2+b+c=0
\]
\[
  x = \frac{-b \pm \sqrt{b^2-4ac}}{2a}.
\]
\[
  \sum_a\underline c\dot bc'
\]
\end{document}