### tetrahedron ABCD : circumsphere and insphere

a = (a_{x}, a_{y}, a_{z}) ... d = (d_{x}, d_{y}, d_{z}) are the coordinates of the vertices,

N_{abc} is a cross product, thus a vector orthogonal to the plane (ABC).

The coordinates of the centers S_{C} of the circumsphere and S_{I} of the insphere,

just as their respective radii R and r, express themselves with determinants and norms.