sin^2(x)=(1-cos2x)/2=1/2-(1/2)cos2x ...(1)由于：cosx=1-(x^2)/2!+(x^4)/4!-(x^6)/6!+...有：cos2x=1-(2x)^2/2!+(2x)^4/4!-(2x)^6/6!+...(2)将（2）代入（1）得：sin^2(x)=1/2-(1/2)cos2x=(1/2)[(2x)^2/2!-(2x)^4/...