a+bi=r * e^(iA)
c+di= r' * e^(iC)
==>(a+bi)(c+di)=r * e^(iA)*r' * e^(iC)=(r r')*e^(i(A+C))
(a+bi)/(c+di)=r * e^(iA)*r' * e^(iC)=(r/r')*e^(i(A-C))
A乘(除)以B表示B对A的长度和辐角的作用.