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.
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.
equation together with aligned, as in \begin{equation} \label{eqn:eqlabel} \begin{aligned}[b] f(x) &= x^2 , \\ g(x) &= \exp( x ) . \end{aligned} \end{equation}. Note the [b] after the {aligned}: this puts the equation number on the bottom line (more specifically, it aligns the baseline of the bottom of the entire aligned environment with the baseline of whatever comes before and after it in the equation, such as the equation number).
Apr 6, 2017 at 9:11
You can use an aligned or gathered environment inside an equation:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\label{eq:t}
\begin{aligned}
f(x) &= x^2\\
g(x) &= \exp(x)
\end{aligned}
\end{equation}
\begin{equation}
\label{eq:u}
\begin{gathered}
f(x) = x^2\\
g(x) = \exp(x)
\end{gathered}
\end{equation}
\end{document}
