推導:. cos2A=cos (A+A)=cosAcosA-sinAsinA=cos^2A-sin^2A=2cos^2A-1=1-2sin^2A. 正切二倍角公式:. tan2α=2tanα/ [1- (tanα)^2] tan (1/2*α)= (sinα)/ (1+cosα)= (1-cosα)/ ...
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