A sphere is inscribed in tetrahedron ABCD and is tangent to faces BCD,CAD,ABD,ABC at points P,Q,R,S respectively. Segment PT is the sphere's diameter, and lines TA,TQ,TR,TS meet the plane BCD at points A′,Q′,R′,S′. respectively. Show that A is the center of a circumcircle on the triangle S′Q′R′. geometry3D geometryspheretetrahedroncircumcirclegeometry unsolved