使用神秘代数结论(读者自证不难:P),可以得到利用重心坐标系的极简纯tikz版本:
\documentclass[tikz,border=3.14pt]{standalone}
\usepackage{fourier}
\begin{document}
\begin{tikzpicture}
\coordinate (A) at (0,0);
\coordinate (B) at (5,0);
\coordinate (C) at (1,3);
\draw (A) -- (B) -- (C) -- cycle;
\draw[semithick,magenta]
plot[domain=0:360,samples=144,smooth]
(
barycentric cs:A={1/3+cos(\x)/3},B={1/3+cos(\x-120)/3},C={1/3+cos(\x+120)/3}
);
\node[below left] at (A) {$A$};
\node[below right] at (B) {$B$};
\node[above] at (C) {$C$};
\end{tikzpicture}
\end{document}



















问 如何绘制斯坦纳内切椭圆?