cos(α+兀／6)－sinα=2√3／3
√3/2cosa-1/2sina-sina=2√3/3
√3/2cosa-3/2sina=2√3/3
1/2cosa-√3/2sina=2/3
sin兀／6cosa-cos兀／6sina=2/3
sin(兀／6-a)=2/3
sin(a-兀／6)=-2/3
sin(a-7兀／6)=2/3