原式=2/(1-tanθ+cotθ-cotθtanθ)
=2/(1-sinθ/cosθ+cosθ/sinθ-1)
=2/[(cos²θ-sin²θ)/sinθcosθ]
=2sinθcosθ/(cos²θ-sin²θ)
=sin2θ/cos2θ
=tan2θ