解：cos(π/6-a)=√3/3,∴cos(5π/6+a)=cos〔π-(5π/6-a)〕=-cos(π/6-a)=-√3/3sin²(a-π/6)=1-cos²(a-π/6)=1-cos(π/6-a)=1-（-√3/3珐罚粹核诔姑达太惮咖）2=1-1/3=2/3cos(5π/6+a)-sin²(a-π/6)=-√3/3-2/3
