用逆推法证明：2/(tanα-cotα)=sin2α/{(2sin^2)α-1}2/(sina/cosa-cosa/sina)=2sinacosa/(-cos2a)1/[(sin²a-cos²a)/(sinacosa)]=sinacosa/(-cos2a)sinacosa/(-cos2a)=sinacosa/(-cos2a)恒成立以上各步可...