有第一条件,求出sinα=4√3/7 cosα=1/7
cos(α+β)=cosαcosβ- sinαsinβ = -11
又 sinβ^2+cosβ^2 = 1;
求出 cosβ = 1/2
sinβ = -√3/2
β = 240°
2β = 480°
sin(α+2β) = sinαcos2β+cosαsin2β = -11/14