If z1,z2,z3 be three complex numbers which are in H.P. And the points A(z1),B(z2),C(z3) are non-collinear, and O is origin, then:
O, A, B, C are concyclic
z2 = 2z1z3z1+z3
z2−0z1−0 = ∣∣z2z1∣∣eiα
∣∣z3z1−z3∣∣eiβ = 12∣∣z2z1∣∣eiβ
∴arg ⎧⎩z2z1⎫⎭ = arg⎧⎩z2−z3z1−z3⎫⎭
⇒∠AOB = ∠ACB
Hence, O,A,B,C are concyclic.