triangle

A triangle is a polygon with three corners and three sides.

\documentclass[tikz,margin={2cm 0cm}]{standalone}
\usepackage{tkz-euclide}
%
\begin{document}
\begin{tikzpicture}
    \tkzDefPoint(0,0){A}
    \tkzDefPoint(3,0){B}
    \tkzDefPoint(1,2){C}
	%
    \tkzDrawPolygon[thick](A,B,C)
\end{tikzpicture}
\end{document}

special triangles:
equilateral triangle
isosceles triangle
scalene triangle
right triangle

triangle inequality

trigonometry
law of sines
law of cosines