如何在 Tikz 中绘制弧线？

Question

可能重复：
创建此图像的最佳方式是正方形和角度

如何在线 OA 和 OB 之间画一条圆弧来表示一个角度？

\begin{figure}
{
\begin{tikzpicture}[scale=2]
\node [draw, thick, minimum size=3.8cm, circle] at (0,2) {};
\node [draw, thick, minimum size=3cm, circle] at (1.3,2) {};
\draw (0,2) -- (1.3,2);
\draw (0,2) -- (0.8,2.56);
\draw (0,2) -- (0.8,1.44);
\draw (0,2) -- (0,2.95);
\draw (0-0.1,2.5) node{$a$};
\draw(1.3,2)--(1.770,2.6);
\draw (1.4,2.3) node{$r$};
\draw (0-0.1,2) node{$O$};
\draw (0.8-0.1,2.56) node{$A$};
\draw (0.8-0.1,1.44) node{$B$};
\end{tikzpicture}
}
\caption{}
\end{figure}

Answer 1

\begin{tikzpicture}[scale=2]
\node [draw, thick, minimum size=3.8cm, circle] at (0,2) {};
\node [draw, thick, minimum size=3cm, circle] at (1.3,2) {};
\draw (0,2) -- (1.3,2);
\draw (0,2) -- (0.8,2.56);
\draw (0,2) -- (0.8,1.44);
\draw (0,2) -- (0,2.95);
\draw (0-0.1,2.5) node{$a$};
\draw(1.3,2)--(1.770,2.6);
\draw (1.4,2.3) node{$r$};
\draw (0-0.1,2) node{$O$};
\draw (0.8-0.1,2.56) node{$A$};
\draw (0.8-0.1,1.44) node{$B$};
% This is the angle: Take that part of circle which comes inside this rectangle
\path[clip]  (0,1.86) -- (0.4,1.86) -- (0.4,2.14) -- (0,2.14) -- cycle;
% The circle itself
\node [draw, blue,  thick, minimum size=1cm, circle] at (0,2) {};
\end{tikzpicture}