17. In AABC, a median is drawn from vertex A to the midpoint of BC, which is labelled D. If AB = b and AC = 7, prove that AD = ,b + 57. F A VECTOR BY A SCALAR NEL 18. Let POR be a triangle in which M is the midpoint of PQ and N is the midpoint of PR. If PM = a and PN = b, find vector expressions for MN and QR in terms of a and b. What conclusions can be drawn about MN and OR? Explain. P M . Q R