sin(5π/6-2a)=sin(π-5π/6+2a)=sin(2a+π/6)=cos(π/2-2a-π/6)=cos(π/3-2a)=cos[2(π/6-a)]
=2cos²(π/6-a)-1=2×（√3/3）²-1=2/3-1=-1/3