\documentclass{article}
\usepackage{tikz}
\begin{document}
% Radius of regular polygons
\newdimen\R
\R=2cm
\begin{tikzpicture}
% Indicate the boundary of the regular polygons
\draw[very thick] (0:\R) \foreach \x in {120,240} {
-- (\x:\R)
} -- cycle (90:\R) node[above] {} ;
\draw[xshift=5.0\R,very thick] (0:\R) \foreach \x in {72,144,...,359} {
-- (\x:\R)
} -- cycle (90:\R) node[above] {} ;
\end{tikzpicture}
\end{document}