(a+bi)*(c+di)
=ac+adi+bci+bd*i^2
=(ac-bd)+(ad+bc)i
(a+bi)÷(c+di)
=(a+bi)(c-di)÷[(c+di)(c-di)]
=(ac-adi+bci-bdi^2)÷(c^2-d^2i^2)
=[(ac+bd)+(bc-ad)i]/(c^2+d^2)