[Tex/LaTex] label multiple lines of equations

equationslabels

Suppose I have the equations

\begin{align}
 f(x) = x^2
 g(x) = \exp( x )
\end{align}

I would like to use on single label for these lines, and ideally the equation number would be placed vertically between these two lines.

Although this question appears very basic, I have not found an explanation neither via search engines, nor on this website.

Best Answer

The very thing you want is the split environment, provided by the amsmath class.

Sample code.

\documentclass{article}
\usepackage[paperwidth=90mm]{geometry}% <-- better demonstrates the vertical alignment
\usepackage{amsmath}
\begin{document}

Eqn.~\eqref{eqn:eqlabel} has a single label split
across the two equations, as you can see here:
\begin{align}
\label{eqn:eqlabel}
\begin{split}
 f(x) &= x^2 ,
\\
 g(x) &= \exp( x ) .
\end{split}
\end{align}

\end{document}

Result.

Sample document using the split environment

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