答： sin(π/6-a)=1/3 sin(a-π/6)=-1/3 cos(2π/3+2a) =cos(π-π/3+2a) =-cos(π/3-2a) =-cos[2(a-π/6)] =-1+2*{sin(a-π/6) ]^2 =-1+2*(-1/3)^2 =-1+2/9 =-7/9
