tanθ[(1-sinθ)/(1+cosθ)][(1+sinθ)/(1-cosθ)]
=tanθ([1-(sinθ)^2]/[1-(cosθ)^2])
=tanθ[(cosθ)^2)/(sinθ)^2)]
=(sinθ/cosθ)[(cosθ)^2)/(sinθ)^2)]
=cosθ/sinθ
=cotθ
即得tanθ[(1-sinθ)/(1+cosθ)]=cotθ[(1-cosθ)/(1+sinθ)]