cosα=-4/5,α是第三象限的角
则sinα=-根号(1-cos²α)=-3/5
tan（α/2+π/4）
=tan[(α+π/2)/2]
=sin(α+π/2)/[1+cos(α+π/2)]
=cosα/(1-sinα)
=-4/5÷(1+3/5)
=-1/2