Switch between leqno and reqno options (of amsmath) in the same document

Question

I'd like to know if it is possible switch between the leqno and reqno options in the same document keeping the numbering of the equations. Namely i use the reqno option (default for amsmath and the standard article class), but i want that some equations are numbered by the left side (as if I had relied the leqno option) and the others no.

I'd like a commands as \leqnomode and \reqnomode to change the position of \tag in anywhere of the document.

Is this possible?

Answer 1

Here you go:

\documentclass{article}
\usepackage[leqno]{amsmath}

\makeatletter
\newcommand{\leqnomode}{\tagsleft@true}
\newcommand{\reqnomode}{\tagsleft@false}
\makeatother
\begin{document}

\begin{align}
  f(x) &= ax^2 + bx + c \\
  g(x) &= dx^2 + ex + f
\end{align}

\reqnomode

\begin{align}
  f(x) &= ax^2 + bx + c \\
  g(x) &= dx^2 + ex + f
\end{align}

\end{document}