Tetrahedral volume = 1/3 (bottom area) * high

If Tetrahedral volume corresponds to Pv, Tetrahedral volume (Tv)=Pv/6

(x1,y1,z1) is acme P

(x2,y2,z2) is acme Q

(x3,y3,z3) is acme R

(x4,y4,z4) is acme S。