我想在一个多行方程周围放置一个方框。最小工作示例：
\documentclass{article}
\usepackage[utf8]{inputenc}
%\usepackage{amsmath}
\usepackage{comment}
\usepackage{mathtools, cuted}
%\usepackage{natbib}
\usepackage{amssymb}
\usepackage{breqn}
%\usepackage{amsthm}
\usepackage{hyperref}
%\bibliographystyle{plain}
%\newtheorem{thm}{Theorem}[section]
%\newtheorem{lem}[thm]{Lemma}
%\newtheorem{cor}[thm]{Corollary}
\newtheorem{rmk}{Remark}
%\newtheorem{def}[rmk]{Definition}
%\newtheorem*{proof}{Proof}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{enumitem}
%\usepackage[dvips]{epsfig} % or this line, depending on which
% you prefer.
\makeatletter
\DeclareRobustCommand{\qed}{%
\ifmmode % if math mode, assume display: omit penalty etc.
\else \leavevmode\unskip\penalty9999 \hbox{}\nobreak\hfill
\fi
\quad\hbox{\qedsymbol}}
\newcommand{\openbox}{\leavevmode
\hbox to.77778em{%
\hfil\vrule
\vbox to.675em{\hrule width.6em\vfil\hrule}%
\vrule\hfil}}
\newcommand{\qedsymbol}{\openbox}
\newenvironment{proof}[1][\proofname]{\par
\normalfont
\topsep6\p@\@plus6\p@ \trivlist
\item[\hskip\labelsep\itshape
#1.]\ignorespaces
}{%
\qed\endtrivlist
}
\newcommand{\proofname}{Proof}
\makeatother
\begin{document}
\begin{align}
&\frac{1}{\min_i \sum_j e^{c(i,j)}p(j|i)}\sum_{i,j=1}^s e^{c(i,j)}p(j|i)x_i y_j \left[c(i,j) + \ln p(j|i)-\ln \sqrt{\pi_i} -
\ln\phi^{k(i)}(i) -\ln\left(\sum_{l=1}^s \phi^{k(i)}(l)\pi_la_{lj} \right)+\ln \sqrt{\pi_j}+ \right. \nonumber \\
& \left. \ln\left(\sum_{m=1}^s{\phi^{k(i)}(m)}^2\pi_m\right)\right] \label{first}.
\end{align}
\end{document}
答案1
像这样吗？
\documentclass{article}
\usepackage[margin=0.5in]{geometry}
\usepackage{empheq}
\begin{document}
\begin{empheq}[box=\fbox]{align}
&\frac{1}{\min_i \sum_j e^{c(i,j)}p(j|i)}\sum_{i,j=1}^s e^{c(i,j)}p(j|i)x_i y_j \left[c(i,j) + \ln p(j|i)-\ln \sqrt{\pi_i} -
\ln\phi^{k(i)}(i) -\ln\left(\sum_{l=1}^s \phi^{k(i)}(l)\pi_la_{lj} \right)+\ln \sqrt{\pi_j}+ \right. \nonumber \\
& \left. \ln\left(\sum_{m=1}^s{\phi^{k(i)}(m)}^2\pi_m\right)\right] \label{first}.
\end{empheq}
\end{document}