(cos2x-sin2x)/[(1-cos2x)(1-tan2x)] =cos2x/(1-cos2x)=[cosx)^2-(sinx)^2]/2(sinx)^2 这步是怎么换算的?

问题描述:

(cos2x-sin2x)/[(1-cos2x)(1-tan2x)] =cos2x/(1-cos2x)=[cosx)^2-(sinx)^2]/2(sinx)^2 这步是怎么换算的?

(cos2x-sin2x)/[(1-cos2x)(1-tan2x)]=cos2x[1-(sin2x/cos2x)]/[(1-cos2x)(1-tan2x)] (分母部分提出cos2x)=cos2x(1-tan2x)/[(1-cos2x)(1-tan2x)]=cos2x/(1-cos2x) (分子分母约去(1-tan2x))=[(cosx)^2-(sinx)^2]/2...