(c) Consider p, q, r, s, m and n are the position vectors of P, Q, R, S, M and N be respectively. As, M and N are mid-points of PQ and RSrespectively So, m =
p+q
2
and n =
r+s
2
...(i) Now, PS + QR = s - p + r - q = (r + s) (p + q) = 2n - 2m = 2MN [From eq. (i)]