Determine whether the points A = (1, 2, 3), B = (1, 1, 1), C = (1, 0, 2), and D = (2,-2, 0) are coplanar and find the volume of the tetrahedron with vertices ABCD. My professor did this problem in class as a review for an upcoming test and he didn't get the answer that was on the key. He just chuckled and went on but I would like to know how to really do this problem. The answer should be 5/6 and the points are not coplanar. My teacher got the answer of 3. He did the cross product of a and b times c. He got the cross product of a and b to be -3 and multiplying -3 by c resulted in -3. He used the volume of a tetrahedron to be (1/6)ha, h being C and area being a cross b.