降幂公式

(cosα)^2=(1+cos2α)/2

(sinα)^2=(1-cos2α)/2

(tanα)^2=(1-cos(2α))/(1+cos(2α))推导公式如下

直接运用二倍角公式就是升幂，将公式Cos2α变形后可得到降幂公式：

cos2α=(cosα)^2－(sinα)^2=2(cosα)^2－1=1－2(sinα)^2

cos2α=2(cosα)^2－1,(cosα)^2=(cos2α+1)/2

cos2α=1－2(sinα)^2,(sinα)^2=(1-cos2α)/2