答案1
像这样吗?
\documentclass[twocolumn]{article}
\usepackage{amsmath} % for '\text' macro
\newcommand\mcr[1]{\multicolumn{1}{@{}r@{}}{#1}} % handy shortcut macro
\begin{document}
\begin{equation}
\medmuskip=2mu % reduce spacing around binary operators
\thickmuskip=3mu % reduce spacing around relational operators
\renewcommand\arraystretch{1.5}
R(p_i\mid G) =
\left\{\begin{array}{@{}l@{}}
\textit{ep}_{p_i} + (1- \textit{UA}(p_i\mid G)) ({r_G - \textit{ep}_{p_i}})\\
\mcr{\text{if $\textit{ep} \leq r_G$}} \\[1ex]
\textit{ep}_{p_i} + \textit{UA}(p_i\mid G) ({r_G - \textit{ep}_{p_i}}) \\
\mcr{\text{ if $\textit{ep} > r_G$}}
\end{array}\right.
\label{eq:dis}
\end{equation}
\end{document}