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\begin{frame}
\frametitle{Derivative as a Function}

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A function $f$ is \emph{differentiable at $a$} if $f'(a)$ exists.
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\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}
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Note that the interval $(a,b)$ may be $(-\infty,b)$, $(a,\infty)$ or $(-\infty,\infty)$.\\
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\end{frame}