5. Forces F, = 1(3i-2j+k) N and F, = /(i+ j+3k) N, where 1 and a are scalars, act on a box. Prove that it is not possible for their resultant force to act in the direction of k. [6] 2 6. The points P, Q and R have position vectors and (a) Show that the triangle PQR is isosceles. (b) M is the midpoint of QR. Find the position vector of M. (c) Calculate the area of the triangle PQR. -11 -6 7. The points A, B, C and D have position vectors 4 and 4 20 10 (a) Write down the vector AB. [1] (b) Calculate the length BC. [2] (c) Prove that ABCD is a rhombus. [5]