LaTex cases Howto?

Question

beginner here (downloaded LaTex today) so please be kind. I am trying to get 'cases' to work, but it doesn't seem to understand "&" to separate the equation from the description, nor "\ \" to split the rows. I'm thinking I need to download something extra for these to work? Otherwise I get:

\documentclass{article}
\begin{document}
$$
1/S=\begin{cases}
1/P-1/P_{\oplus} & inferior \\ 
1/P_{\oplus }-1/P & superior 
\end{cases}
$$
\end{document}

resulting in: "Misplaced alignment tab character &."

Without the '&'s I get the two equations on one row instead of separated.

Thanks in advance.

Answer 1

Load the amsmath package:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
1/S=\begin{cases}
1/P-1/P_{\oplus} & \text{inferior} \\
1/P_{\oplus }-1/P & \text{superior} 
\end{cases}
\]
\end{document}