已知cos(a-π/6)=1/3，则sin(2a+π/6)=？

sin(2a+ π/6)
=cos[π/2-(2a+π/6)]
=cos(π/3 -2a)
=cos[2(a- π/6)]
=2cos²(a- π/6)-1
=2×⅓²-1
=-7/9