\pgfmathparse{2}      \let\R\pgfmathresult;
\pgfmathparse{1.2*\R} \let\RR\pgfmathresult;
\pgfmathparse{20}     \let\AngleP\pgfmathresult;
\pgfmathparse{70}     \let\AngleQ\pgfmathresult;


\coordinate (O) at (0,0);
\coordinate (P) at (\AngleP:\R);
\coordinate (Q) at (\AngleQ:\R);

\draw (O) circle(\R);


\filldraw (O) circle(1pt);
\filldraw (P) circle(1pt);
\filldraw (Q) circle(1pt);
\draw [-Latex,very thin] (-\RR,0) -- (\RR,0) node [right] {$x$};
\draw [-Latex,very thin] (0,-\RR) -- (0,\RR) node [above] {$y$};


\draw [-Latex] (O) -- (P);
\draw [-Latex] (O) -- (Q);

\draw (\AngleP:\RR) node {$\theta_1$};
\draw (\AngleQ:\RR) node {$\theta_2$};

\tkzMarkAngle[size=0.75](P,O,Q);
\tkzLabelAngle[pos=0.95](P,O,Q) {$\theta$};

\draw (0,-1.25*\RR) node {$\theta=\theta_2-\theta_1$};
