26. Find a unit vector whose representations are perpendicular to the plane containing PQ and PR if PQ is a representa- tion of the vector i + 3j - 2k and PR is a representation of the vector 2i - j - k. 27. Given the points P(5, 2, -1), Q(2, 4,-2), and R(11, 1, 4). Find a unit vector whose representations are perpendicular to the plane through points P. Q, and R. 28. Find the volume of the parallelepiped having edges PQ, PR, and PS if the points P. Q. R, and S are, respectively, (1. 3, 4). (3, 5, 3). (2, 1, 6), and (2, 2, 5). 29. Find the volume of the parallelepiped PQRS if the vectors V(PQ). V(PR), and V(PS) are, respectively, i + 3j + 2k. 2i + j - k, and i - 2j + k