How to manipulate font size “within” an align environment

Question

I'm using the align package for multiple-row equations. Some rows the equations are just a bit longer than the textwidth (I'm using double column). I only want to reduce the fontsize of those particular rows, not the whole align environment. How do I do that?

Answer 1

The mathtools package provides an \Aboxed macro that boxes an equation inside align environments (and family) across &.
The original code is:

\newcommand\Aboxed[1]{\let\bgroup{\romannumeral-`}\@Aboxed#1&&\ENDDNE}
\def\@Aboxed#1&#2&#3\ENDDNE{%
  \ifnum0=`{}\fi \setbox \z@
    \hbox{$\displaystyle#1{}\m@th$\kern\fboxsep \kern\fboxrule }%
    \edef\@tempa {\kern  \wd\z@ &\kern -\the\wd\z@ \fboxsep
        \the\fboxsep \fboxrule \the\fboxrule }\@tempa \boxed {#1#2}%
}

I’m going to adapt this by measuring the whole width of the equation and re-sizing the whole line to \linewidth. This’ll work with twocolumn, too, but showframe doesn’t show the inner column separator.

In the examples one can see that it works best when the one long line has on both sides of & the longest part of all lines.

The equal sign don’t align perfectly because the left part deos’nt end at the left side of = (and neither in the middle) but to the left from the additional space that is inserted before the relation sign (i.e. \thickmuskip) which will be scaled as well.

Code

\documentclass[
%   twocolumn,
]{article}
\usepackage{amsmath}
\usepackage{graphics}
\usepackage{pgf}
\makeatletter
\newdimen\width@one
\newdimen\width@two
\newdimen\width@onetwo
\newdimen\left@kern
\newcommand\Rboxed[1]{\let\bgroup{\romannumeral-`}\@Rboxed#1&&\ENDDNE}
\def\@Rboxed#1&#2&#3\ENDDNE{%
  \ifnum0=`{}\fi%
  \setbox0\hbox{$\displaystyle#1{}\m@th$}%
  \setbox1\hbox{$\displaystyle{}#2\m@th$}%
  \width@one=\wd0%
  \width@two=\wd1%
  \width@onetwo=\width@one%
  \advance\width@onetwo by \width@two%
  \pgfmathsetmacro{\width@factor}{\linewidth/\width@onetwo}%
  %%% Solution A
%  \pgfmathsetlength{\left@kern}{\width@factor*\width@one}%
%  \edef\@tempa{\kern\left@kern&\kern-\the\left@kern}\@tempa%
%  \scalebox{\width@factor}{$\displaystyle#1{}$}&\scalebox{\width@factor}{$\displaystyle{}#2$}%
%  \resizebox\linewidth!{$#1#2$}%
  %% Solution B
  \edef\@tempb{\noexpand\scalebox{\width@factor}{$\displaystyle#1{}$}&\noexpand\scalebox{\width@factor}{$\displaystyle{}#2$}}%
  \@tempb%
}
\makeatother
%\usepackage[showframe,pass]{geometry}
\usepackage{showframe}
\begin{document}
\begin{align*}
a^2 + b^2 & = c^2 \\
\Rboxed{long equation + very long equation + long equation + long equation + long equation & = a^2+b^2} \\
c^2 & = a^2 + b^2
\end{align*}

\begin{align*}
a^2 + b^2 & = c^2 \\
\Rboxed{long equation + very long equation + long equation + long equation & = a^2+b^2 + long equation} \\
c^2 & = a^2 + b^2
\end{align*}

\twocolumn
\begin{align*}
a^2 + b^2 & = c^2 \\
\Rboxed{long equation + long equation + long & = a^2+b^2} \\
c^2 & = a^2 + b^2
\end{align*}

\begin{align*}
a^2 + b^2 & = c^2 \\
\Rboxed{long equation + long equation + long & = a^2+b^2 + long equation} \\
c^2 & = a^2 + b^2
\end{align*}
\end{document}

Output