sin(α+π/3)+sinα=-4√3/5
sinαcosπ/3+cosαsinπ/3+sinα=-4√3/5
3/2*sinα+√3/2*cosα=-4√3/5
3/2*sinα=-√3/2*cosα-4√3/5
两边平方
9/4*sin²α=9/4(1-cos²α)=3/4*cos²a+12/5*cosx+48/25
cos²α+4/5*cosα-11/100=0
α是第四象限
cosα>0
cosα=(-4+3√3)/5