∵cos(a+π/6)一sina=cosa×√3/2一sina×1/2一sina= cosa×√3/2一sina×3/2=2√3/3,∴sin(a一7π/6)=sina×(一√3/2)一cosa(-1/2)=√3/3(cosa×√3/2一sina×3/2)=√3/3×2√3/3=2/3,