α为第三象限角
则sinα=-√(1-cos²α)=-2√2/3
sin(α+β)=1,∴cos(α+β)=0
cos(2α+β) =cosc[α+(α+β)]
=cosαcos(α+β)-sinαsin(α+β)
=0+2√2/3x1=2√2/3