答案1
起点:
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\def\radius{.5cm}
\begin{document}
\begin{tikzpicture}
\coordinate (a) at (0,0);
\coordinate (b) at (3,-1);
\coordinate (c) at (2,2);
\foreach \nd in {a, b, c} {\draw (\nd) circle (\radius);}
\draw ($(a)!\radius!-90:(b)$) -- ($(b)!\radius!90:(a)$) ($(b)!\radius!-90:(c)$) -- ($(c)!\radius!90:(b)$) ($(c)!\radius!-90:(a)$) -- ($(a)!\radius!90:(c)$) ($(a)!\radius!-90:(b)$);
\draw[dashed] (a) -- (b) -- (c) -- cycle;
\end{tikzpicture}
\end{document}