Consider arg(z4z1)+arg(z3z2) =arg(z1)−arg(z4)+arg(z2)−arg(z3) =(arg(z1)+arg(z2))−(arg(z3)+arg(z4))given(z2=zˉ1 z4=zˉ3)=(arg(z1)+arg(zˉ1))−(arg(z3)+arg(zˉ3)){also(arg(zˉ1)=−arg(z1)arg(zˉ3)=−arg(z3)}=(arg(z1)−arg(z1))−(arg(z3)−arg(z3))=0−0=0