If the points represented by complex numbers z1 = a+ib,z2 = a1+ib1 and z1−z2 are coliinear, then
By definition, z1 = x1+iy1;z2 = x2+iy2;z3 = x3+iy3 are collinear, if
∣∣ ∣∣x1y11x2y21x3y31∣∣ ∣∣ = 0 ⇒ ∣∣ ∣∣ab1a′b′1a−a′b−b′1∣∣ ∣∣ = 0
⇒ab′ = a′b