288. LaTeX技巧369：数学专题之上下划线 widebar

\documentclass{article}
\usepackage{amsmath}
\usepackage{accents}
\usepackage{statex}
\makeatletter
\def\wideubar{\underaccent{{\cc@style\underline{\mskip10mu}}}}
\def\Wideubar{\underaccent{{\cc@style\underline{\mskip8mu}}}}
\makeatother
\makeatletter
\def\widebar{\accentset{{\cc@style\underline{\mskip10mu}}}}
\def\Widebar{\accentset{{\cc@style\underline{\mskip8mu}}}}
\makeatother
\begin{document}

\begin{align*}
\bar{X}_i
\overline{X}_i
\widebar{X}_i
\Widebar{X}_i
\b{X_i}
\ol{X_i}
\end{align*}

\begin{align*}
\underline{X}_i
\wideubar{X}_i
\Wideubar{X}_i
\ul{X}_i
\end{align*}

\end{document}