To find the circumscribing polyhedron
Here, as John Sharp showed, we need the radius of the sphere touching
the faces of the polyhedron we are seeking, the in-radius, which, following
Coxeter [4], we denote by . We are interested in two possible polyhedra:
that with the -gon as face, and that with the triangle as face belonging to
the dual polyhedron. Which triangle depends on which enantiomorph we
have, but the choice does not affect the measurements. It is immediate that
for a -gon , and for a
triangle . If the edge of the circumscribing polyhedron is
, we need to find this from our knowledge of the radius of its shared insphere.
The polyhedron has -gonal faces and -gonal vertex figures.
If is its centre,