降冪公式
(cosA)^2=(1+cos2A)/2
(sinA)^2=(1-cos2A)/2
(tanA)^2=(1-cos(2α))/(1+cos(2α))推導公式如下
直接運用二倍角公式就是升冪,將公式Cos2α變形後可得到降冪公式:
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