18. Given the two unit vectors A - fi+lj-$k and B=-fi+ ij+ik If 6 is the radian measure of the angle between A and B, find sin # in two ways: (a) by using the cross product (formula (3) of this section); (b) by using the dot product and a trigonometric identity. 19. Follow the instructions of Exercise 18 for the two unit vectors: v3 k and B- 3V/3 3V/3 3V/3 20. Show that the quadrilateral having vertices at (1, 1, 3), (-2, 1,-1). (-5, 4, 0), and (-8,4,-4) is a parallelogram and find its area. 21. Show that the quadrilateral having vertices at (1. -2, 3), (4, 3, -1), (2, 2. 1), and (5, 7, -3) is a parallelogram and find its area