Compile equations used in a document to an equation sheet?

Question

I am trying to find a way to take all of the numbered equations in my document (i.e. anything inside an equation environment) and automatically generate an equation sheet appended to the end of the document.

I have found a couple of answered questions (such as here and here) where the tocloft package is used to generate a table-of-contents style reference list of the equations used in the document. This is not what I want to do. What I would like is a new page appended to the end of my document which has the actual equations typeset with their corresponding equation numbers next to them, preferably with the option to put the sheet into 2- or 3-column format.

Answer 1

\equationpage simply lists the equations using equation environments. You can apply any column or font formatting you want.

Note: the equation* environment will not be saved for the list.

\documentclass{article}
\usepackage{mathtools}
\usepackage{environ}

\newcounter{eqsaved}
\newcounter{eqused}

\let\oldequation=\equation% save original equation environment
\let\oldendequation=\endequation

\NewEnviron{myequation}{%
  \oldequation\BODY\oldendequation
  \stepcounter{eqsaved}%
  \global\expandafter\edef\csname myequationnumber\theeqsaved\endcsname{\theequation}%
  \global\expandafter\edef\csname myequation\theeqsaved\endcsname{\BODY}}

\let\equation=\myequation% replace equation environment
\let\endequation=\endmyequation

\newcommand{\equationpage}%
{\ifnum\value{eqused}<\value{eqsaved}\relax
  \loop\stepcounter{eqused}%
    \oldequation{\csname myequation\theeqused\endcsname}
    \tag{\csname myequationnumber\theeqused\endcsname}\oldendequation
  \ifnum\value{eqused}<\value{eqsaved}\relax\repeat
\fi}

\begin{document}
\begin{equation} x=a \end{equation}
\begin{equation} y=b \end{equation}

\newpage\equationpage
\end{document}