Latex角标(subscript/superscript)

\documentclass[UTF8]{ctexart}

\usepackage{mathtools}
\usepackage{amsmath}
\usepackage{mhchem}
\usepackage{tensor}

\begin{document}

%For general math scripts before something, use \prescript provided by mathtools package, for better alignment
\begin{equation}
	\prescript{14}{2}{\mathbf{C}}	
\end{equation}

%For large operators, use \sideset from amsmath	
\begin{equation}
	\sideset{_a^b}{'}\sum A_n
\end{equation}

%For chemical equations, use mhchem package
\begin{equation}
	\ce{^{227}_{90}Th+}
\end{equation}

%For tensors, use tensor package
\begin{equation}
	BIOR(f, \chi)=\frac{\sum_{\tensor[]{f}{^'} \in \chi} \Delta l(f, \tensor[]{f}{^'})}{L(f)}	
\end{equation}
	
\begin{equation}
\tensor*[^A]{P}{^B} = \tensor*[^B]{P}{}\tensor*[^A_B]{T}{}
\end{equation}

\end{document}