已知sin(α+π/3)+sinα=-(4/5)√3，-π/2<α<0，则cos(α+2π/3)=?解：sin(α+π/3)+sinα=2sin(α+π/6)cos(π/6)=(√3)sin(α+π/6)=-(4/5)√3∴sin(α+π/6)=-4/5cos(α+2π/3)=cos[π/2+(α+π/6)]=-sin(α+π/6)=4/5，故应选D。
题呢
