%% 
%%  An UIT Edition example
%% 
%%  Example 03-06-3 on page 28.
%% 
%%  Copyright (C) 2010 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 
%% 
%% ==== 
% Show page(s) 1
%% 
\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{375.57637pt}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\setcounter{equation}{31}
\renewcommand\theequation{3.\arabic{equation}}\setlength\parindent{0pt}
\setlength\parskip{1ex plus 0.2ex}
\StartShownPreambleCommands
\usepackage{color,calc}
\makeatletter
\def\myMathBox{\@ifnextchar[{\my@MBoxi}{\my@MBoxi[black]}}
\def\my@MBoxi[#1]{\@ifnextchar[{\my@MBoxii[#1]}{\my@MBoxii[#1][white]}}
\def\my@MBoxii[#1][#2]#3#4{%
  \par\noindent\fcolorbox{#1}{#2}{%
     \parbox{\linewidth-1.5\labelwidth-2\fboxrule-2\fboxsep}{#3}}%
  \parbox{1.5\labelwidth}{\begin{eqnarray}\label{#4}\end{eqnarray}}\par}
\makeatother
\StopShownPreambleCommands
\begin{document}
Similar to inline mode, displayed formulae can be framed, but have to be \ldots
(This text is to illustrate the line width.)
%
\begin{equation}
  f(x)=x^2 +x
\end{equation}
%
\myMathBox[red]{ \[f(x)=x^2 +x \]}{eq:frame3}
\myMathBox[red][yellow]{ \[f(x)=x^2 +x\] }{eq:frame4}
\myMathBox{ \[f(x)=x^2 +x\] }{eq:frame5}
\end{document}