cos(a-π/6)=cosa * cosπ/6 + sina * sinπ/6 ={[根号(3)]/2}*cosa + (1/2) * sina =1/3；sin(2π/3-a)= sin2π/3 * cosa - cos2π/3 * sina ={[根号(3)]/2}*cosa + (1/2) * sina因此sin(2π/3-a)= 1/3；呵呵，是-1/3，都有功亏一篑啊呵呵……，没仔细看，sorry，那最后的答案是-1/3。