Given a sphere \gamma in space, a plane (\mathscr{P}) tangent to \gamma at a point P. Points A,B,C and D lie on (\mathscr{P}). Take point A' such that tetrahedron A'BCD is circumscribed about \gamma, define B',C',D' similarly. Prove that points A',B',C',D' are coplanar and the plane passing through them is tangent to \gamma.