Draw a incircle given the length of the three sides of the triangle.

incircle.tex

\documentclass{article}% Typeset by pdfLaTeX
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \def\a{2}
  \def\b{3}
  \def\c{4}
  \def\B{acos((\c^2+\a^2-\b^2)/(2*\a*\c))}
  \def\r{\a*\c*sin(\B)/(\a+\b+\c)}
  \coordinate (B) at (0,0);
  \coordinate (C) at (\a,0);
  \coordinate (A) at ({\B}:\c);
  \coordinate (incenter) at (barycentric cs:A=\a,B=\b,C=\c);
  \draw (A)--(B)--(C)--cycle;
  \draw (incenter) circle[radius={\r}];
\end{tikzpicture}
\end{document}
%
% Reference
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%   PGF Manual 139-140
%   https://pgf-tikz.github.io/pgf/pgfmanual.pdf
%   (accessed 2021-06-05)
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%   GitHub - mebiolabo/EulerLine
%   https://github.com/mebio-labo/EulerLine
%   (accessed 2021-06-05)
%