降幂公式
(cosα)^2=(1+cos2α)/2
(sinα)^2=(1-cos2α)/2
(tanα)^2=(1-cos2α)/(1+cos2α)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