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)
