If obj is a sphere, radius(obj) returns the radius of obj.

If obj is a regular polyhedron or an archimedean solid, radius(obj) returns the radius of the circum-sphere which passes through all the vertices.

If obj is a dual of an archimedean solid, radius(obj) returns the radius of the in-sphere which touches all the faces.

The command with(geom3d,radius) allows the use of the abbreviated form of this command.

$\mathrm{with}\left(\mathrm{geom3d}\right)\:$

$\mathrm{sphere}\left(s\,x^2+y^2+z^2=2\,\left[x\,y\,z\right]\right)$

$s$

$\mathrm{radius}\left(s\right)$

$\sqrt{2}$

$\mathrm{tetrahedron}\left(t\,\mathrm{point}\left(o\,0\,0\,0\right)\,1\right)$

$t$

$\mathrm{radius}\left(t\right)$

$1$