令a=θ/2左边=tana-1/tana=(tan²a-1)/tana=1/[tana/(tan²a-1)]=-2/[2tana/(1-tan²a)]=-2/tan2a=-2/tanθ=右边命题得证
令a=θ/2左边=tana-1/tana=(tan²a-1)/tana=1/[tana/(tan²a-1)]=-2/[2tana/(1-tan²a)]=-2/tan2a=-2/tanθ=右边