答案1
PSTricks 解决方案仅用于娱乐或比较目的。
这个答案是几年前就准备好的。
\documentclass[pstricks]{standalone}
\usepackage{pst-eucl}
\begin{document}
\begin{pspicture}(7,5)
\pstGeonode[PointName=none,PointSymbol=none](1,4){A}(3,1){B}(6,1){C}
\psline{<->}(A)(B)(C)
\pstMarkAngle[MarkAngleRadius=1,LabelSep=.5]{C}{B}{A}{$120^\circ$}
\end{pspicture}
\end{document}
比例注
答案2
\documentclass[margin=3mm, tikz]{standalone}
\usetikzlibrary{angles,
quotes}
\begin{document}
\begin{tikzpicture}
\coordinate (O);
\draw[red,->] (O) -- (4,0) coordinate[label=below:${x}$] (x);
\draw[blue,->] (O) -- (-4,2) coordinate[label=above:${y}$] (q);
%
\pic [draw, thick, <->,
angle radius=16mm,angle eccentricity=1.1,"$\theta$"] {angle = x--O--q};
\end{tikzpicture}
\end{document}