Suppose z1, z2, z3 are three complex numbers, and Δ=14i1z1z¯11z2z¯21z2z¯3 then
Re (∆) = 0
Im (∆) = 0
Re (∆) ≥ 0
Im (∆) ≤0
Δ¯=1−4i1z¯1z11z¯2z21z¯3z3=Δ
⇒∆ is purely real ⇒ Im(∆) = 0