# Problem with gather and labels

This gives a way of combining two different math environments (so there is no extra vertical skip):

But add labels to that exsample

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{gather}
\begin{split}
1 + 1 = 2\\
1 * 2 * 3 * 4 * 5 * 6 * 7 = 7!
\end{split}\\
\begin{align}
7 + 5 &= 12\label{a}\\
12 &= 11 + 1\label{b}
\end{align}
\end{gather}

\end{document}


and a problem occurs:

! Package amsmath Error: Multiple \label's: label 'a' will be lost.

What to do?

Use aligned, together with the trick in the previous answer:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
\multispan{2}\hfill{\begin{aligned} & 1 + 1 = 2\\ & 1 * 2 * 3 * 4 * 5 * 6 * 7 = 7! \end{aligned}}\hfill \label{A} \\
7 + 5 &= 12+1-1+1-1+1-1+1-1 \label{B} \\
12 &= 11 + 1 \label{C}
\end{align}
Equations \eqref{A}, \eqref{B} and \eqref{C}

\end{document} You can not use align inside gather ... With combination of align and split as follows:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{align}
\begin{split}
1 + 1 & = 2\\
1 * 2 * 3 * 4 * 5 * 6 * 7 & = 7!
\end{split}                         \label{eq:a}   \\
7 + 5 & = 12    \label{eq:b}   \\
12 & = 11 + 1\label{eq:c}
\end{align}
Equations \eqref{eq:a}, \eqref{eq:b} and \eqref{eq:c} \dots
\end{document} 