原式= cos(α+π)sin(α)=------------------------ cos(α)[-cos(α)] sin(α)=------------ (∵-π/2<α<0∴cos(α)>0且sin(α)<0 , ∴原式符号为负) cos(α) 1-cos²(α)=--------------- cos(α) 1-1/9=-(--------) 1/3=-8/3
