【参考答案】 ∵(5π/6) -a=(π/3) -a+(π/2)∴cos(5π/6 -a)=cos[(π/3 -a)+(π/2)]=-sin[(π/3)-a]=-1/3
-1/3，cos(5π/6-α)=cos(π/2+π/3-α)=-sin(π/3-α)=-1/3
