6. Let T be the topological boundary of the tetrahedron in Exercise 5, with outward pointing normal, and 8 be the surface obtained by taking away the slanted face from T; i.e., 8 has three triangular faces, one each in the planes x = 0, y = 0, z = O. If 88 is oriented positively, prove for all C1 functions P, Q, R : 8 -T R that las P dx + Q dy + R dz = I Is (Ry - Q z) dy dz + (Pz - Rx) dz dx + (Q x - Py) dx dy.