证明：
(sinα+cosα-1)(sinα-cosα+1)/sin2α
=(sin²α-(cosα-1)²)/sin2α
=(1-cos²α - cos²α+2cosα-1)/(2sinαcosα)
=2cosα(1-cosα)/(2sinαcosα)
=(1-cosα)/sinα
=2sin²(α/2)/(2sinα/2 cosα/2)
=sinα/2 / cosα/2
=tan α/2