![我怎样才能写出这个等式?](https://linux22.com/image/355347/%E6%88%91%E6%80%8E%E6%A0%B7%E6%89%8D%E8%83%BD%E5%86%99%E5%87%BA%E8%BF%99%E4%B8%AA%E7%AD%89%E5%BC%8F%EF%BC%9F.png)
答案1
\documentclass{article}
\usepackage{amsmath}
\usepackage{accents}
\newcommand{\Underbar}[1]{\underaccent{\bar}{#1}}
\newcommand{\ubar}{\underbar u}
\begin{document}
\begin{equation}
\Underbar{\Underbar\varepsilon} =
\frac{1}{2} (\accentset{*}{\ubar})
\end{equation}
\end{document}