17. What does the set of points equidistant from three lines containing the edges of a trihedral angle and lying inside this angle represent?
17. The plane of symmetry of edges $a$ and $b$ intersects the plane of symmetry of edges $b$ and $c$ along a line $S$, which is the set of points equidistant from the lines containing the three edges of the given trihedral angle. (Note that the plane of symmetry of edges $c$ and $a$ also passes through $S$.)