\begin{frame}
  \frametitle{Functions}
  
  \begin{example}
    The area $A$ of a circle depends on its radius $r$.
    The rule is
    $$A = \pi r^2$$
    We say that $A$ is a \emph{function} of $r$.
  \end{example}
  \bigskip
  \begin{minipage}{.49\textwidth}
  \begin{center}
    \begin{tikzpicture}[default,nodes={scale=.8,inner sep=1mm}]
      \draw[fill=cblue!10] (0,0) circle (1.5cm);
      \node[cblue] at (0,-.75cm) {area $A = \pi r^2$};
      \draw[cred,dotted] (0,0) -- node[above] {radius $r$} (1.5cm,0mm);
      \draw[fill=black] (0,0) circle (.5mm);
    \end{tikzpicture}
  \end{center}
  \end{minipage}
  \pause
  \begin{minipage}{.49\textwidth}
    \begin{center}
    \scalebox{.7}{
    \begin{tikzpicture}[default]
      \def\diax{r}
      \def\diay{A}
      \diagram{-1}{4}{-1}{4}{1}
      \diagramannotatez
      \diagramannotatexx{1/1 cm,2/2 cm,3/3 cm}
      \diagramannotateyy{1/10 cm$^2$,2/20 cm$^2$,3/30 cm$^2$}
      \draw[cblue] plot[smooth,domain=0:3.6,samples=20] (\x,{.1*pi*\x^2});
    \end{tikzpicture}
    }
    \end{center}
  \end{minipage}
\end{frame}