%% 
%%  An UIT Edition example
%% 
%%  Example 10-09-13 on page 222.
%% 
%%  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}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\usepackage{amsmath} \newcommand*\CMD[1]{\texttt{\textbackslash#1}}
\newcommand*\aboxed[2]{\rlap{\boxed{#1#2}}\phantom{\hskip\fboxrule\hskip\fboxsep#1}}
\StopShownPreambleCommands
\begin{document}
\begin{align*}
  F &= \vec\nabla\Phi \\
    &= \frac{\partial\Phi}{\partial x}\hat\imath + [\ldots] \\
    &= (3x^3+y)\hat\imath + [\ldots] \\
  \aboxed{F}{= 20\hat\imath + [\ldots]}    \tag*{\CMD{aboxed}}\\
  \boxed{F=20\hat\imath + [\ldots]}        \tag*{\CMD{boxed}}\\
  \rlap{\boxed{F=20\hat\imath + [\ldots]}} \tag*{\CMD{rlap}\{\CMD{boxed}\}}\\
  \aboxed{F}{= 20\hat\imath + [\ldots]}    \tag*{\CMD{aboxed}}\\
  \ldots &= \ldots
\end{align*}
\end{document}