cos（5/6*π+a）=-cos[π-(5/6*π+a)]=-cos[π/6-a]
-(sin[a-π/6])^2=(cos(a-π/6))^2-1
cos（π/6-a）=cos(a-π/6)
所以
cos（5/6π+a）-(sin(a-π/6))^2
= -√3/3+1/3-1
= -(2+√3)/3