[Tex/LaTex] Adding letters to equation numbers

equationsnumbering

I want to number use a equation and add a letter to the equation number such as:

f(x) =a x and g(x) = b x                     (3.1.1a,b)

That is e.g. using \begin{equation} … \label{eq} \end{equation}

Best Answer

The amsmath package provides \tag{<whatever>} that allows you to tag (or number) an equation with <whatever> you want. Here's a minimal working example:

enter image description here

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\[
  f(x)=ax \qquad \text{and} \qquad g(x)=bx \tag{3.1.1a,b}\label{myeq}
\]
For example, see \eqref{myeq}.
\end{document}

If you don't want the parentheses around the \tag, use \tag* instead. With this, however, you can only reference the entire equation, not the separate components.

For the ability to reference either the main equation or the sub-equations as a whole, you could use the following setup:

enter image description here

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\renewcommand{\theequation}{\thesubsection.\arabic{equation}}
\begin{document}
\setcounter{section}{2}% Just for illustrative purposes
\section{A section} \subsection{A subsection}
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae, felis.%
\refstepcounter{equation}\label{myeqn1}% Correctly mark and label equation
\[
  f(x)=ax \qquad \text{and} \qquad g(x)=bx \tag{\theequation a,b}\label{myeqn2}
\]
For example, see~\eqref{myeqn1} and~\eqref{myeqn2}.
\end{document}​

The idea is to correctly step and reference (using \refstepcounter) the equation before it is typeset, and use the equation number in \tag (via \theequation) for appropriate labelling and referencing.

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