Anonymous 1123
At [here](https://topanswers.xyz/tex?q=1233), I used Mathematica to find four vertices of a tetrahedron and draw Circumscribed sphere.
Now I try to draw circumscribed sphere a tetrahedron knowing length of six edges. How can I do this? 
user 3.14159
The six edges do not specify the tetrahedron uniquely. Apart from the orientation in 3d, you'd also make one discrete choice. So I need to make some assumptions. The lengths of the edges are stored in functions on l1, l2 etc.
1. The first three vertices are taken to be in the xy plane. The first edge is along the x axis. This fixes the orientation of the tetrahedron up to discrete choices (signs).
2. The fourth vertex is computed as the intersection of three spheres of radii l4, l5 and l6 using a pgf version [this code](https://stackoverflow.com/a/18654302).

The 3dtools is in principle prepared to deal with arbitrary orientations. However, this would require more input from your side. As long as you only draw the tetrahedron, you do not have to worry about this because you can adjust the view to get the orientation you want (but there are still discrete choices). An improved version of the code that draws a face may go into the 3dtools library so that one could, at some point, draw arbitrary polyhedra.


\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{3dtools}% https://github.com/marmotghost/tikz-3dtools
% \usetikzlibrary{backgrounds}
\makeatletter
% something like this may go into the 3dtools library
\tikzset{3d/.cd,polyhedron/.cd,
draw face with corners/.code={\begingroup\c@pgf@counta0\relax
\pgfutil@for\pgfutil@tmpa:={#1}\do{%
\pgfmathsetmacro\pgfutil@tmpT{TD("\pgfutil@tmpa")}%
\ifcase\c@pgf@counta
\pgfmathsetmacro\pgfutil@tmpA{TD("\pgfutil@tmpa")}%
\or
\pgfmathsetmacro\pgfutil@tmpB{TD("\pgfutil@tmpa")}%
\or
\pgfmathsetmacro\pgfutil@tmpC{TD("\pgfutil@tmpa")}%
\fi
\ifnum\c@pgf@counta=1\relax
\edef\pgfutil@tmpl{(\pgfutil@tmpT)}%
\else
\edef\pgfutil@tmpl{\pgfutil@tmpl -- (\pgfutil@tmpT)}%
\fi
}%
\ifnum\c@pgf@counta<3\relax
\typeout{A face needs at least three vertices. However, only \the\c@pgf@counta\space vertices were specified.}%
\else
\edef\pgfutil@tmpO{\pgfkeysvalueof{/tikz/3d/polyhedron/O}}%
\pgfmathsetmacro\pgfutil@tmpO{TD("\pgfutil@tmpO")}%
\edef\pgfutil@tmpL{\pgfkeysvalueof{/tikz/3d/polyhedron/L}}%
\pgfmathsetmacro\pgfutil@tmpL{TD("\pgfutil@tmpL")}%
\pgfmathsetmacro{\pgfutil@tmpb}{TD("(\pgfutil@tmpB)-(\pgfutil@tmpA)x(\pgfutil@tmpC)-(\pgfutil@tmpA)")}%
\pgfmathsetmacro{\pgfutil@tmpc}{TD("(\pgfutil@tmpA)-(\pgfutil@tmpO)")}%
\pgfmathtruncatemacro{\pgfutil@tmpd}{sign(TD("(\pgfutil@tmpb)o(\pgfutil@tmpc)"))}%
\ifnum\pgfutil@tmpd=-1\relax
\pgfmathsetmacro{\pgfutil@tmpb}{TD("(\pgfutil@tmpB)-(\pgfutil@tmpA)x(\pgfutil@tmpA)-(\pgfutil@tmpC)")}%
\fi
\pgfmathsetmacro{\pgfutil@tmpe}{screendepth(\pgfutil@tmpb)}%
\pgfmathsetmacro{\pgfutil@tmpf}{sqrt(TD("(\pgfutil@tmpb)o(\pgfutil@tmpb)"))}%
\pgfmathsetmacro{\pgfutil@tmpg}{sqrt(TD("(\pgfutil@tmpL)o(\pgfutil@tmpL)"))}%
% \typeout{polyhedron face}
% \typeout{(A)=\pgfutil@tmpA,(B)=\pgfutil@tmpB,(C)=\pgfutil@tmpC}
% \typeout{(N)=\pgfutil@tmpb}
% \typeout{b=\pgfutil@tmpb,f=\pgfutil@tmpf,g=\pgfutil@tmpg}
% \typeout{boundary: \pgfutil@tmpl}
\pgfmathsetmacro{\pgfutil@tmph}{TD("(\pgfutil@tmpL)o(\pgfutil@tmpb)")/\pgfutil@tmpf/\pgfutil@tmpg}%
\pgfmathtruncatemacro{\pgfutil@tmpi}{70+30*\pgfutil@tmph}%
% \typeout{(\pgfutil@tmpb),\pgfutil@tmpe}
\ifdim\pgfutil@tmpe pt<0pt\relax
\else
\draw[fill=tikz@td@face@color!\pgfutil@tmpi!black] \pgfutil@tmpl -- cycle;
\fi
\fi
\endgroup
},
color/.code={\colorlet{tikz@td@face@color}{#1}},color=yellow,
O/.initial={(0,0,0)},% point inside the polyhedron
L/.initial={(1,1,1)},% "light source"
}
% temporary
% conceivably a more developed version will go into 3dtools
\tikzset{pics/generic tetrahedron/.style={code={\c@pgf@counta0\relax
\pgfutil@for\pgfutil@tmpa:={#1}\do{%
\ifcase\c@pgf@counta
\edef\pgfutil@tmpA{\pgfutil@tmpa}%
\or
\edef\pgfutil@tmpB{\pgfutil@tmpa}%
\or
\edef\pgfutil@tmpC{\pgfutil@tmpa}%
\or
\edef\pgfutil@tmpD{\pgfutil@tmpa}%
\fi
}%
% \typeout{A=\pgfutil@tmpA,B=\pgfutil@tmpB,C=\pgfutil@tmpC,D=\pgfutil@tmpD}
\ifnum\c@pgf@counta=4\relax
\tikzset{3d/polyhedron/.cd,
draw face with corners={{\pgfutil@tmpA},{\pgfutil@tmpB},{\pgfutil@tmpC}},
draw face with corners={{\pgfutil@tmpA},{\pgfutil@tmpB},{\pgfutil@tmpD}},
draw face with corners={{\pgfutil@tmpA},{\pgfutil@tmpC},{\pgfutil@tmpD}},
draw face with corners={{\pgfutil@tmpB},{\pgfutil@tmpC},{\pgfutil@tmpD}}
}
\else
\typeout{A tetrahedron needs 4 vertices. However, you specified \the\c@pgf@counta\space vertices.}%
\fi
}}}
\makeatother
\begin{document}
\begin{tikzpicture}[scale=3,
dot/.style={circle,inner sep=1pt,fill},
declare function={l1=1;l2=1.1;l3=1.2;l4=1.3;l5=1.4;l6=1;}]
% ^^^ the lengths of the tetrahedron
\begin{scope}[3d/install view={phi=100,psi=0,theta=70}]
% construct a triangle of three vertices in the xy plane
% using the cosine law
\pgfmathsetmacro{\mytheta}{acos((l3*l3-l1*l1-l2*l2)/(2*l1*l2))}
\path (0,0,0) coordinate (v1)
(l1,0,0) coordinate (v2)
({l2*cos(\mytheta)},{l2*sin(\mytheta)},0) coordinate (v3);
% we do not need this here but in principle 3dtools is prepared to
% deal with arbitrarily oriented tetrahedra.
% \path[overlay,3d/define orthonormal dreibein={A={(v1)},B={(v2)},C={(v3)}}];
% however, the following works for the above input
% this is a dictionary for the code at https://stackoverflow.com/a/18654302
\tikzset{declare function={d=l1;i=l2*cos(\mytheta);j=l2*sin(\mytheta);
r1=l4;r2=l5;r3=l6;}}
\pgfmathsetmacro{\myx}{(r1*r1-r2*r2+d*d)/(2*d)}
\pgfmathsetmacro{\myy}{(r1*r1-r3*r3+2*i*\myx+j*j)/(2*j)}
\pgfmathsetmacro{\tmp}{r1*r1-\myx*\myx-\myy*\myy}
\ifdim\tmp pt<0pt
\typeout{Spheres do not intersect.}
\else
\pgfmathsetmacro{\myz}{sqrt(\tmp)}
\path (\myx,\myy,\myz) coordinate (v4);
\fi
% an educated guess for a coordinate inside the polyhedron
\path[overlay,3d coordinate={(O')=0.25*(v1)+0.25*(v2)+0.25*(v3)+0.25*(v4)}];
\path[3d/polyhedron/O={(O')}] pic[transform shape]{%
generic tetrahedron={{(v1)},{(v2)},{(v3)},{(v4)}}};
\path[3d/circumsphere center={A={(v1)},B={(v2)},C={(v3)},D={(v4)}}]
coordinate (I);
\end{scope}
\pgfmathsetmacro{\csr}{sqrt(TD("(v1)-(I)o(v1)-(I)"))}
\foreach \XX in {1,...,4}
{\pgfmathsetmacro{\csr}{sqrt(TD("(v\XX)-(I)o(v\XX)-(I)"))}
\typeout{\csr}}
%  \path foreach \X in {1,...,4}
%  {(v\X) coordinate[dot,label=above:{$v_\X$}]}
%  (I) (I) coordinate[dot,label=above:{$I=\pgfmathparse{TD("(I)")}% % (\pgfmathprintvector\pgfmathresult)^T$}];
\end{tikzpicture}
\end{document}


![Screen Shot 2020-09-24 at 4.37.21 PM.png](/image?hash=217360ff98241371ea4e96bee34a3d2f695cd3d6b8de62d0e415dac70013481c)

This can be animated as usual.

Temporary addendum: this code is merely to test things. I uses (but does not yet rely on) an addendum to the library 3dtools: intersection of three spheres. I plan to add the polyhedron face style at a given point when it is clearer what the expected functionality is.


\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{3dtools}% https://github.com/marmotghost/tikz-3dtools
\usetikzlibrary{backgrounds}
\makeatletter
% something like this may go into the 3dtools library
\tikzset{3d/.cd,polyhedron/.cd,
draw face with corners/.code={\begingroup\c@pgf@counta0\relax
\pgfutil@for\pgfutil@tmpa:={#1}\do{%
\pgfmathsetmacro\pgfutil@tmpT{TD("\pgfutil@tmpa")}%
\ifcase\c@pgf@counta
\pgfmathsetmacro\pgfutil@tmpA{TD("\pgfutil@tmpa")}%
\or
\pgfmathsetmacro\pgfutil@tmpB{TD("\pgfutil@tmpa")}%
\or
\pgfmathsetmacro\pgfutil@tmpC{TD("\pgfutil@tmpa")}%
\fi
\ifnum\c@pgf@counta=1\relax
\edef\pgfutil@tmpl{(\pgfutil@tmpT)}%
\else
\edef\pgfutil@tmpl{\pgfutil@tmpl -- (\pgfutil@tmpT)}%
\fi
}%
\ifnum\c@pgf@counta<3\relax
\typeout{A face needs at least three vertices. However, only \the\c@pgf@counta\space vertices were specified.}%
\else
\edef\pgfutil@tmpO{\pgfkeysvalueof{/tikz/3d/polyhedron/O}}%
\pgfmathsetmacro\pgfutil@tmpO{TD("\pgfutil@tmpO")}%
\edef\pgfutil@tmpL{\pgfkeysvalueof{/tikz/3d/polyhedron/L}}%
\pgfmathsetmacro\pgfutil@tmpL{TD("\pgfutil@tmpL")}%
\pgfmathsetmacro{\pgfutil@tmpb}{TD("(\pgfutil@tmpB)-(\pgfutil@tmpA)x(\pgfutil@tmpC)-(\pgfutil@tmpA)")}%
\pgfmathsetmacro{\pgfutil@tmpc}{TD("(\pgfutil@tmpA)-(\pgfutil@tmpO)")}%
\pgfmathtruncatemacro{\pgfutil@tmpd}{sign(TD("(\pgfutil@tmpb)o(\pgfutil@tmpc)"))}%
\ifnum\pgfutil@tmpd=-1\relax
\pgfmathsetmacro{\pgfutil@tmpb}{TD("(\pgfutil@tmpB)-(\pgfutil@tmpA)x(\pgfutil@tmpA)-(\pgfutil@tmpC)")}%
\fi
\pgfmathsetmacro{\pgfutil@tmpe}{screendepth(\pgfutil@tmpb)}%
\pgfmathsetmacro{\pgfutil@tmpf}{sqrt(TD("(\pgfutil@tmpb)o(\pgfutil@tmpb)"))}%
\pgfmathsetmacro{\pgfutil@tmpg}{sqrt(TD("(\pgfutil@tmpL)o(\pgfutil@tmpL)"))}%
\pgfmathsetmacro{\pgfutil@tmph}{TD("(\pgfutil@tmpL)o(\pgfutil@tmpb)")/\pgfutil@tmpf/\pgfutil@tmpg}%
\pgfmathtruncatemacro{\pgfutil@tmpi}{70+30*\pgfutil@tmph}%
\ifdim\pgfutil@tmpe pt<0pt\relax
\scoped[on background layer]{%
\draw[fill=tikz@td@face@color!\pgfutil@tmpi!black,3d/polyhedron/back] \pgfutil@tmpl -- cycle;}
\else
\draw[fill=tikz@td@face@color!\pgfutil@tmpi!black,3d/polyhedron/fore] \pgfutil@tmpl -- cycle;
\fi
\fi
\endgroup
},
color/.code={\colorlet{tikz@td@face@color}{#1}},color=yellow,
O/.initial={(0,0,0)},% point inside the polyhedron
L/.initial={(1,1,1)},% "light source",
fore/.style={draw,solid},
back/.style={draw,dashed,fill=none},
}%
% temporary
% conceivably a more developed version will go into 3dtools
\tikzset{pics/generic tetrahedron/.style={code={\c@pgf@counta0\relax
\pgfutil@for\pgfutil@tmpa:={#1}\do{%
\ifcase\c@pgf@counta
\edef\pgfutil@tmpA{\pgfutil@tmpa}%
\or
\edef\pgfutil@tmpB{\pgfutil@tmpa}%
\or
\edef\pgfutil@tmpC{\pgfutil@tmpa}%
\or
\edef\pgfutil@tmpD{\pgfutil@tmpa}%
\fi
}%
% \typeout{A=\pgfutil@tmpA,B=\pgfutil@tmpB,C=\pgfutil@tmpC,D=\pgfutil@tmpD}
\ifnum\c@pgf@counta=4\relax
\tikzset{3d/polyhedron/.cd,
draw face with corners={{\pgfutil@tmpA},{\pgfutil@tmpB},{\pgfutil@tmpC}},
draw face with corners={{\pgfutil@tmpA},{\pgfutil@tmpB},{\pgfutil@tmpD}},
draw face with corners={{\pgfutil@tmpA},{\pgfutil@tmpC},{\pgfutil@tmpD}},
draw face with corners={{\pgfutil@tmpB},{\pgfutil@tmpC},{\pgfutil@tmpD}}
}
\else
\typeout{A tetrahedron needs 4 vertices. However, you specified \the\c@pgf@counta\space vertices.}%
\fi
}}}
\makeatother
\begin{document}
\foreach\Angle in {5,15,...,355}
{\begin{tikzpicture}[dot/.style={circle,inner sep=1pt,fill},
declare function={a=3;l1=a;l2=a*sqrt(3);l3=2*a;l4=a*sqrt(2);l5=a;l6=a*sqrt(5);}]
% ^^^ the lengths of the tetrahedron
\begin{scope}[3d/install view={phi=\Angle,psi=0,theta=70}]
% construct a triangle of three vertices in the xy plane
% using the cosine law
\pgfmathsetmacro{\mytheta}{acos((l3*l3-l1*l1-l2*l2)/(2*l1*l2))}
\path (0,0,0) coordinate (v1)
(l1,0,0) coordinate (v2)
({l2*cos(\mytheta)},{l2*sin(\mytheta)},0) coordinate (v3);
% we do not need this here but in principle 3dtools is prepared to
% deal with arbitrarily oriented tetrahedra.
% \path[overlay,3d/define orthonormal dreibein={A={(v1)},B={(v2)},C={(v3)}}];
% however, the following works for the above input
% this is a dictionary for the code at https://stackoverflow.com/a/18654302
\tikzset{declare function={d=l1;i=l2*cos(\mytheta);j=l2*sin(\mytheta);
r1=l4;r2=l5;r3=l6;}}
\pgfmathsetmacro{\myx}{(r1*r1-r2*r2+d*d)/(2*d)}
\pgfmathsetmacro{\myy}{(r1*r1-r3*r3+2*i*\myx+j*j)/(2*j)}
\pgfmathsetmacro{\tmp}{r1*r1-\myx*\myx-\myy*\myy}
\ifdim\tmp pt<0pt
\typeout{Spheres do not intersect.}
\else
\pgfmathsetmacro{\myz}{sqrt(\tmp)}
\path (\myx,\myy,\myz) coordinate (v4);
% an educated guess for a coordinate inside the polyhedron
%   \path[3d/plane through={(v1) and (v2) and (v3) named pABC}];
%   \path[3d/project={(v4) on pABC}] coordinate (v4');
\path[overlay,3d coordinate={(O')=0.25*(v1)+0.25*(v2)+0.25*(v3)+0.1*(v4)}];
\path[3d/polyhedron/O={(O')},3d/polyhedron/fore/.append style={opacity=0.7}]
pic[transform shape]{%
generic tetrahedron={{(v1)},{(v2)},{(v3)},{(v4)}}};
\path[3d/circumsphere center={A={(v1)},B={(v2)},C={(v3)},D={(v4)}}]
coordinate (I);
\path[3d/intersection of three spheres={A={(v1)},B={(v2)},C={(v3)},
rA=l4,rB=l5,rC=l6}];
\pgfmathsetmacro{\myv}{TD("(v4)")}
\pgfmathsetmacro{\myIa}{TD("(i1)")}
\pgfmathsetmacro{\myIb}{TD("(i2)")}
\pgfmathsetmacro{\myex}{TD("(ex)")}
\pgfmathsetmacro{\myey}{TD("(ey)")}
\pgfmathsetmacro{\myez}{TD("(ez)")}
%    \typeout{(ex)=(\myex),(ey)=(\myey),(ez)=(\myez)}
%    \typeout{(v4)=(\myv),(i1)=(\myIa),(i2)=(\myIb)}
\fi
\end{scope}
\pgfmathsetmacro{\dAB}{sqrt(TD("(v1)-(v2)o(v1)-(v2)"))}
\pgfmathsetmacro{\dAC}{sqrt(TD("(v1)-(v3)o(v1)-(v3)"))}
\pgfmathsetmacro{\dBC}{sqrt(TD("(v2)-(v3)o(v2)-(v3)"))}
\pgfmathsetmacro{\dBD}{sqrt(TD("(v2)-(v4)o(v2)-(v4)"))}
\pgfmathsetmacro{\dCD}{sqrt(TD("(v3)-(v4)o(v3)-(v4)"))}
\pgfmathsetmacro{\lA}{l1}
\pgfmathsetmacro{\lB}{l2}
\pgfmathsetmacro{\lC}{l3}
\pgfmathsetmacro{\lD}{l4}
\pgfmathsetmacro{\lE}{l5}
\pgfmathsetmacro{\lF}{l6}
% check the lengths of the edges
\pgfmathsetmacro{\csr}{sqrt(TD("(v1)-(I)o(v1)-(I)"))}
%  \foreach \XX in {1,...,4}
%  {\pgfmathsetmacro{\csr}{sqrt(TD("(v\XX)-(I)o(v\XX)-(I)"))}
% %  \pgfmathsetmacro{\myd}{TD("(v\XX)-(I)"))}
% %  \pgfmathsetmacro{\csr}{sqrt(TD("(\myd)o(\myd)"))}
%  \typeout{\csr}}
%  \path foreach \X in {1,...,4}
%  {(v\X) coordinate[dot,label=above:{$v_\X$}]}
%  (I) (I) coordinate[dot,label=above:{$I=\pgfmathparse{TD("(I)")}% % (\pgfmathprintvector\pgfmathresult)^T$}];
\end{tikzpicture}}
\end{document}


![ani.gif](/image?hash=6c3634162346b12d3c06f01048bb4db89546ffb443462fa8e3aa82e2545f6698)

This version uses the new key intersection of three spheres that was added to 3dtools.

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{3dtools}% https://github.com/marmotghost/tikz-3dtools
\usetikzlibrary{backgrounds}
\makeatletter
% something like this may go into the 3dtools library
\tikzset{3d/.cd,polyhedron/.cd,
draw face with corners/.code={\begingroup\c@pgf@counta0\relax
\pgfutil@for\pgfutil@tmpa:={#1}\do{%
\pgfmathsetmacro\pgfutil@tmpT{TD("\pgfutil@tmpa")}%
\ifcase\c@pgf@counta
\pgfmathsetmacro\pgfutil@tmpA{TD("\pgfutil@tmpa")}%
\or
\pgfmathsetmacro\pgfutil@tmpB{TD("\pgfutil@tmpa")}%
\or
\pgfmathsetmacro\pgfutil@tmpC{TD("\pgfutil@tmpa")}%
\fi
\ifnum\c@pgf@counta=1\relax
\edef\pgfutil@tmpl{(\pgfutil@tmpT)}%
\else
\edef\pgfutil@tmpl{\pgfutil@tmpl -- (\pgfutil@tmpT)}%
\fi
}%
\ifnum\c@pgf@counta<3\relax
\typeout{A face needs at least three vertices. However, only \the\c@pgf@counta\space vertices were specified.}%
\else
\edef\pgfutil@tmpO{\pgfkeysvalueof{/tikz/3d/polyhedron/O}}%
\pgfmathsetmacro\pgfutil@tmpO{TD("\pgfutil@tmpO")}%
\edef\pgfutil@tmpL{\pgfkeysvalueof{/tikz/3d/polyhedron/L}}%
\pgfmathsetmacro\pgfutil@tmpL{TD("\pgfutil@tmpL")}%
\pgfmathsetmacro{\pgfutil@tmpb}{TD("(\pgfutil@tmpB)-(\pgfutil@tmpA)x(\pgfutil@tmpC)-(\pgfutil@tmpA)")}%
\pgfmathsetmacro{\pgfutil@tmpc}{TD("(\pgfutil@tmpA)-(\pgfutil@tmpO)")}%
\pgfmathtruncatemacro{\pgfutil@tmpd}{sign(TD("(\pgfutil@tmpb)o(\pgfutil@tmpc)"))}%
\ifnum\pgfutil@tmpd=-1\relax
\pgfmathsetmacro{\pgfutil@tmpb}{TD("(\pgfutil@tmpB)-(\pgfutil@tmpA)x(\pgfutil@tmpA)-(\pgfutil@tmpC)")}%
\fi
\pgfmathsetmacro{\pgfutil@tmpe}{screendepth(\pgfutil@tmpb)}%
\pgfmathsetmacro{\pgfutil@tmpf}{sqrt(TD("(\pgfutil@tmpb)o(\pgfutil@tmpb)"))}%
\pgfmathsetmacro{\pgfutil@tmpg}{sqrt(TD("(\pgfutil@tmpL)o(\pgfutil@tmpL)"))}%
\pgfmathsetmacro{\pgfutil@tmph}{TD("(\pgfutil@tmpL)o(\pgfutil@tmpb)")/\pgfutil@tmpf/\pgfutil@tmpg}%
\pgfmathtruncatemacro{\pgfutil@tmpi}{70+30*\pgfutil@tmph}%
\ifdim\pgfutil@tmpe pt<0pt\relax
\scoped[on background layer]{%
\draw[fill=tikz@td@face@color!\pgfutil@tmpi!black,3d/polyhedron/back] \pgfutil@tmpl -- cycle;}
\else
\draw[fill=tikz@td@face@color!\pgfutil@tmpi!black,3d/polyhedron/fore] \pgfutil@tmpl -- cycle;
\fi
\fi
\endgroup
},
color/.code={\colorlet{tikz@td@face@color}{#1}},color=yellow,
O/.initial={(0,0,0)},% point inside the polyhedron
L/.initial={(1,1,1)},% "light source",
fore/.style={draw,solid},
back/.style={draw,dashed,fill=none},
}%
% temporary
% conceivably a more developed version will go into 3dtools
\tikzset{pics/generic tetrahedron/.style={code={\c@pgf@counta0\relax
\pgfutil@for\pgfutil@tmpa:={#1}\do{%
\ifcase\c@pgf@counta
\edef\pgfutil@tmpA{\pgfutil@tmpa}%
\or
\edef\pgfutil@tmpB{\pgfutil@tmpa}%
\or
\edef\pgfutil@tmpC{\pgfutil@tmpa}%
\or
\edef\pgfutil@tmpD{\pgfutil@tmpa}%
\fi
}%
%\typeout{A=\pgfutil@tmpA,B=\pgfutil@tmpB,C=\pgfutil@tmpC,D=\pgfutil@tmpD}
\ifnum\c@pgf@counta=4\relax
\tikzset{3d/polyhedron/.cd,
draw face with corners={{\pgfutil@tmpA},{\pgfutil@tmpB},{\pgfutil@tmpC}},
draw face with corners={{\pgfutil@tmpA},{\pgfutil@tmpB},{\pgfutil@tmpD}},
draw face with corners={{\pgfutil@tmpA},{\pgfutil@tmpC},{\pgfutil@tmpD}},
draw face with corners={{\pgfutil@tmpB},{\pgfutil@tmpC},{\pgfutil@tmpD}}
}
\else
\typeout{A tetrahedron needs 4 vertices. However, you specified \the\c@pgf@counta\space vertices.}%
\fi
}}}
\makeatother
\begin{document}
\foreach\Angle in {155} %{5,15,...,355}
{\begin{tikzpicture}[dot/.style={circle,inner sep=1pt,fill},
declare function={a=3;l12=a;l23=a*sqrt(3);l13=2*a;
l14=a*sqrt(2);l24=a;l34=a*sqrt(5);}]
% ^^^ the lengths of the tetrahedron;
% l12 is the distance between vertex 1 and vertex 2, etc.
\begin{scope}[3d/install view={phi=\Angle,psi=0,theta=70}]
\pgfmathsetmacro{\mytheta}{acos((l23*l23-l12*l12-l13*l13)/(2*l12*l13))}
\pgfmathtruncatemacro{\itest}{(cos(\mytheta)<0)}
\ifnum\itest=1
\pgfmathsetmacro{\mytheta}{180-\mytheta}
\fi
\path (0,0,0) coordinate (v1) (0,0,l12) coordinate (v2)
({l13*sin(\mytheta)},0,{l13*cos(\mytheta)}) coordinate (v3);
\pgfmathtruncatemacro{\itest}{(l14+l24>l12)&&(l14+l34>l13)&&(l24+l34>l23)}
\ifnum\itest<1
\typeout{Not all of the spheres do intersect.}
\else
\path[3d/intersection of three spheres={A={(v1)},B={(v2)},C={(v3)},
rA=l14,rB=l24,rC=l34}];
\pgfmathsetmacro{\myIa}{TD("(i1)")}
\path (\myIa) coordinate (v4);
% an educated guess for a coordinate inside the polyhedron
\path[overlay,3d coordinate={(O')=0.25*(v1)+0.25*(v2)+0.25*(v3)+0.1*(v4)}];
\path[3d/polyhedron/O={(O')},3d/polyhedron/fore/.append style={opacity=0.7}]
pic[transform shape]{%
generic tetrahedron={{(v1)},{(v2)},{(v3)},{(v4)}}};
\path[3d/circumsphere center={A={(v1)},B={(v2)},C={(v3)},D={(v4)}}]
coordinate (I);
\fi
\end{scope}
\pgfmathsetmacro{\csr}{sqrt(TD("(v1)-(I)o(v1)-(I)"))}