答案1
尝试:
\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\usepackage{amsmath}
\numberwithin{equation}{section}
\begin{document}
\section{First}
\begin{equation}
a = b \tag*{(\theequation)$_{\pm}$}\refstepcounter{equation}
\end{equation}
\begin{equation}
c = d \tag*{(\theequation)$_{-}$}\refstepcounter{equation}
\end{equation}
\end{document}