If ⎧⎩3−z12−z1⎫⎭⎧⎩2−z23−z2⎫⎭ = k, then points A(z1), B(z2), C(3,0) and D(2,0) (taken in clockwise sense) will
arg⎧⎩3−z12−z1⎫⎭+arg⎧⎩2−z23−z2⎫⎭
= arg⎧⎩3−z12−z1⎫⎭⎧⎩2−z23−z2⎫⎭
If k > 0, its argument will be zero
So, θ1 & θ2 are equal in magnitude but opposite sign.
So DC subtends equal qngle at A & B. So, points are concyclic if k > 0