\documentclass{beamer}
\title{Math 23a - Proof  }
\author{ }
\date{ }
\begin{document}
\maketitle
\setbeamercovered{dynamic}
\begin{frame}
\begin{itemize}
\item (Statement of all or part of the theorem to be proved)
\pause
\item  (mathematical expression)
\pause
\item   (mathematical expression)

\end{itemize}
\end{frame}

\begin{frame}
\begin{itemize}
\item (Further statement to be proved).
\pause
\item    (mathematical expression)
\item ...
\end{itemize}

\end{frame}






\end{document}
.