泰勒级数展开
e^ix=1+ix+1/2!(ix)^2+1/3!(ix)^3+……+1/n!(ix)^n+……
=(1-1/2!x^2+1/4!x^4-……)+i(x-1/3!x^3+1/5!x^5-……)
=cos(x)+i*sin(x)