\documentclass[a4paper,10pt]{beamer}
\usepackage[utf8]{inputenc}
\usepackage[french]{babel}
\usepackage{graphicx}
\usepackage{thumbpdf}
\usepackage{wasysym}
\usepackage{ucs}
\usepackage{pgf,pgfarrows,pgfnodes,pgfautomata,pgfheaps,pgfshade}
\usepackage{verbatim}
\usepackage{subfig}
\usepackage{amssymb}
\usepackage{tikz}
\usepackage{enumerate}
\usepackage{eurosym}
\usepackage{multicol}


\begin{document}
  \begin{frame}{Méthode par combinaison}

      \begin{center}
        $ \left\{ 
             \begin{array}{lcl}
                 3x + 4y &=& 195 \\
                 4x + 5y &=& 249
             \end{array}
             \right.$
          
      \end{center}

             \vfill

             \onslide<2->{
        Composition de meubles correspondante

        \includegraphics[scale=0.2]{./fig/meubles_t}

        \textbf{Comment faire pour avoir le même nombre de meubles $X$?}
    }
    
             \vfill
  \end{frame}

  \begin{frame}{Éxercices}

      \begin{multicols}{2}

          \begin{enumerate}[a)]
              \item $ \left\{ 
             \begin{array}{lcl}
                 2x + 4y &=& -2\\
                 4x + 5y &=& 5
             \end{array}
             \right.$
             % x = 5 et y = -3

             \vspace{0.5cm}
             \onslide<2>{x=5 et y = -3}
             \vspace{0.5cm}

              \item $ \left\{ 
             \begin{array}{lcl}
                 3x + 2y &=& 75 \\
                 5x + 5y &=& 90
             \end{array}
             \right.$
             % x = -3 et y = 21

             \vspace{0.5cm}
             \onslide<2>{x=-3 et y = 21}
             \vspace{0.5cm}

              \item $ \left\{ 
             \begin{array}{lcl}
                 6x + 3y &=& 114 \\
                 4x + 9y &=& 307
             \end{array}
             \right.$
             %x = 2.5 et y = 33

             \vspace{0.5cm}
             \onslide<2>{x=2.5 et y = 33}
             \vspace{0.5cm}

              \item $ \left\{ 
             \begin{array}{lcl}
                 6 x + 44 y &=& 755 \\
                 30 x - 7y &=& 143
             \end{array}
             \right.$
             %x = 8.5 et y = 16

             \vspace{0.5cm}
             \onslide<2>{x=8.5 et y = 16}
             \vspace{0.5cm}

          \end{enumerate}
          
      \end{multicols}
      
  \end{frame}

\end{document}
