\documentclass[10pt,envcountsect]{beamer}

\mode<presentation> {
  \usetheme{Warsaw}
}

\usepackage[english]{babel}
\usepackage[latin1]{inputenc}
\usepackage{times}
\usepackage[T1]{fontenc}
\usepackage{pstricks}
\usepackage{multicol}
\usepackage{monster2e}


\begin{document}

\title{Episode AP06\\ Introduction to logic}
\subtitle{European section, season 1}
\date{}

\begin{frame}
  \titlepage
\end{frame}

{\Large
\begin{center}
\begin{frame}{Proposition -- Hypothesis -- Conclusion}
\Large

{\red If} it's snowing {\red then} it's cold.
\end{frame}

\begin{frame}{Converse}
\Large

{\red If} it's cold {\red then} it's snowing.
\end{frame}

\begin{frame}{Contrapositive}
\Large

{\red If} it's not cold {\red then} it's not snowing.
\end{frame}


\begin{frame}{A property : a true implication}
\Large

{\red If} $M$ lays on the perpendicular bisector of $AB$,\\
{\red then} the length $AM$ is equal to the length $MB$.\\
\pause

\bigskip

$$M \in med(AB) \Rightarrow AM=MB$$
\end{frame}


\begin{frame}{An equivalence}
The length $AM$ is equal to the length $MB$,\\
{\red if and only if} $M$ lays on the perpendicular bisector of $AB$.\\
\pause

\bigskip

$$M \in med(AB)\Leftrightarrow AM=MB$$
\end{frame}
\end{center}
}
\end{document}
