\usetikzlibrary{arrows.meta, bending, angles, decorations.pathreplacing}


  x=4cm, y=4cm, >=Triangle,
  bracy/.style={decoration={brace, amplitude=2mm}},
  dec/.style={decorate, draw, solid}

% Points P, Q, R, and the origin O.
\coordinate[label=below right:$O$](O) at (0,0)
 coordinate[label=above left:
  {$P\big(\cos (\pi - \theta'), \sin (\pi - \theta')\big)$}]  (P) at (120:1)
 coordinate[label=below left:
  {$Q\big(\cos (\pi + \theta'), \sin (\pi + \theta')\big)$}]  (Q) at (240:1)
 coordinate[label=below right:{$R(1, 0)$}] (R) at (1,0);

% Points for X and Y axis.
\coordinate (x-) at (xyz cs: x=-1.1)
  coordinate (x+) at (xyz cs: x=+1.1)
  coordinate (y-) at (xyz cs: y=-1.1)
  coordinate (y+) at (xyz cs: y=+1.1);

% Section of (the upper half of) the unit circumference 
\draw (-1, 0) arc [radius=1, start angle=180, end angle = -180];

% Mark points P and R.
\fill (P) circle [radius=2pt];
\fill (Q) circle [radius=2pt];
\fill (R) circle [radius=2pt];

% Line from Q to P.
\draw (P) [very thick] -- (Q); % [postaction={dec, bracy, ultra thin}]

% Line from O to Q.
\draw (Q) -- (O); % [postaction={dec, bracy, ultra thin}]

% Line from O to P.
\draw (O) -- (P) ;

% Actually draw X and Y axis.
\draw[->] (x-) -- (x+) ;
\draw[->] (y-) -- (y+) ;

% Arcs for both angles.
\draw[thick, ->](0.5, 0) arc(0:120:0.5) node[midway, above=1mm, right=1mm]{$\pi - \theta'$};
\draw[thick, ->](0.2, 0) arc(0:240:0.2) node[near end, below=5mm, left=-1mm]{$\pi + \theta'$};