\begin{frame}
  \frametitle{Derivative as a Function}
  
  \begin{block}{}
    A function $f$ is \emph{differentiable at $a$} if $f'(a)$ exists.
  \end{block}
  \pause\smallskip
  \begin{block}{}
    A function $f$ is \emph{differentiable on an open interval $(a,b)$}
    if it is differentiable at every number of the interval.    
  \end{block}
  \pause
  Note that the interval $(a,b)$ may be $(-\infty,b)$, $(a,\infty)$ or $(-\infty,\infty)$.\\ 
%   \pause\medskip
\end{frame}