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In Calculus, the default measurement for angles is \emph{radian}.
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\begin{itemize}
\item consider a circle with radius $1$, and
\item<4-> an sector of this circle with angle $\alpha$ (radians)
\end{itemize}
\begin{center}\hspace{2cm}
\begin{tikzpicture}[default,scale=2]
\draw (0,0) circle (1cm);
\draw[<->] (0,0) -- node[right] {radius $1$} (270:1cm);
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\draw[fill=clblue!20] (0,0) to (60:1cm) arc (60:0:1cm) -- cycle;
\draw[->] (.4cm,0) arc (0:60:.4cm);
\node at (30:.55cm) {$\alpha$};
\pause

\draw[cred,line width=1mm] (60:1cm) arc (60:0:1cm);
\pause
\node[cred,anchor=west] at (30:1.05cm) {arc has length $\alpha$};
\end{tikzpicture}
\end{center}
\mpause[0]{
Then the arc of the sector has length $\alpha$ (equal to the angle).
}
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