16. What does the set of points that are equidistant from the planes of all three faces of a trihedral angle and lying inside this angle represent?
16. The bisector half-plane of one of the dihedral angles of the given trihedral angle is the set of points equidistant from the planes of the corresponding faces, for example, the first and second and lying inside this dihedral angle. The bisector half-plane of the second and third faces possesses an analogous property. Having a common point (the vertex $S$), these half-planes intersect along the ray $S A$, which is the set of points equidistant from all three faces (note that therefore the bisector half-plane of the first and third faces also passes through $[S A)$).

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