\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning, calc} 
\tikzset{
    %Define standard arrow tip
    >=stealth',
    % Define arrow style
    pil/.style={
           ->,
           thick}
}

\begin{document}
\begin{preview}
\begin{tikzpicture}[scale=0.8]
    % draw the background
    \draw [line width=0.1pt, fill=gray!2] (0,0) -- (3, 3) -- (5,0) -- cycle;

    % angle gamma
    \begin{scope}[shift={(3, 3)}]
        \draw[fill=green!30] (0,0) -- (-135:.75cm) arc (-135:-55:.75cm);
        \draw (-90:0.5cm) node {$\gamma$};
    \end{scope}

    \coordinate[label=left:$A$] (A) at (0,0);
    \coordinate[label=right:$B$] (B) at (5,0);
    \coordinate[label=above:$C$] (C) at (3,3);
    \
    \draw[->, very thick] (A) to[] node[above] {$\vec c$} (B);
    \draw[->, very thick] (C) to[] node[above] {$\vec b$} (A);
    \draw[->, very thick] (C) to[] node[above=0.1cm] {$\vec a$} (B);

    \coordinate[label=-90:$c$] (c) at ($(A)!0.5!(B)$);
    \coordinate[label=-120:$a$] (a) at ($(B)!0.5!(C)$);
    \coordinate[label=-45:$b$] (b) at ($(C)!0.5!(A)$);
  \end{tikzpicture}
\end{preview}
\end{document}