sin(α+β)cosβ - sinβcos(α+β)=－5/13
sin(α+β-β)=-5/13
sinα=-5/13
α是第三象限角
cosα=-[√(13²-5²)]/13
=-12/13
tan(α/2)
=sinα/(cosα+1)
=(-5/13)/(-12/13+1)
=-5