根号3sin(π/6-a)-cos(π/6-a)
= 2 { 根号3/2 sin(π/6-a) - 1/2 cos(π/6-a) }
= 2 { sin(π/6-a)cosπ/6 - cos(π/6-a)sinπ/6 }
= 2sin(π/6-a-π/6)
= 2sin(-a)
= -2sina