::
Il codice
`\documentclass{minimal}
\usepackage{amsmath}
\begin{document}
\begin{equation}\label{etichetta}
\begin{gathered}
S_{i}=\frac{P_{r}}{4\pi r^{2}}\\S_{i}=\frac{P_{r}}{4\pi r^{2}}
\end{gathered}
\end{equation}
\end{document}`
a me funziona.
Ciao,
L.
