% Drawing angles using the PG 3.0 angles and quotes libraries
% Author: Paul Gaborit
\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{quotes,angles}
\begin{document}
\begin{tikzpicture}
  \draw
    (3,-1) coordinate (a) node[right] {a}
    -- (0,0) coordinate (b) node[left] {b}
    -- (2,2) coordinate (c) node[above right] {c}
    pic["$\alpha$", draw=orange, <->, angle eccentricity=1.2, angle radius=1cm]
    {angle=a--b--c};
\end{tikzpicture}
\end{document}