LaTeX-examples/tikz/triangle-labels/triangle-labels.tex

\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usepackage{tkz-euclide}
\usetikzlibrary{angles,quotes}

\usetkzobj{all}
\usetikzlibrary{shapes, calc, decorations}
\usepackage{amsmath,amssymb}
\usepackage{helvet}
\usepackage[eulergreek]{sansmath}

\begin{document}
\begin{preview}
\begin{tikzpicture}[very thick,font=\sansmath\sffamily]
    \tkzDefPoint(0,0){A}
    \tkzDefPoint(2,0){B}
    \tkzDefPoint(0.5,2.0){C}

    \tkzLabelPoints[below left,font=\sansmath\sffamily](A)
    \tkzLabelPoints[below right,font=\sansmath\sffamily](B)
    \tkzLabelPoints[above,font=\sansmath\sffamily](C)

    % Label lines
    \tkzLabelSegment[right](B,C){$a$}
    \tkzLabelSegment[left](A,C){$b$}
    \tkzLabelSegment[below](A,B){$c$}

    % Draw polygon
    \tkzDrawPolygon[fill=gray!10](A,B,C)

    % Mark angles
    \tkzMarkAngle[arc=l,size=0.6cm,fill=green!30](B,A,C)
    \tkzLabelAngle[pos=0.3](B,A,C){$\alpha$}

    \tkzMarkAngle[arc=l,size=0.6cm,fill=green!30](C,B,A)
    \tkzLabelAngle[pos=0.45](C,B,A){$\beta$}

    \tkzMarkAngle[arc=l,size=0.6cm,fill=green!30](A,C,B)
    \tkzLabelAngle[pos=0.45](A,C,B){$\gamma$}

    % Draw polygon
    \tkzDrawPolygon[very thick](A,B,C)
\end{tikzpicture}
\end{preview}
\end{document}