sin(2a+b)=sin(a+b)*cosa+sina*cos(a+b)；sinb=sin(a+b-a)=sin(a+b)*cosa-cos(a+b)*sina;代入sin（2a+b）+2sinb=0 ：3*sin(a+b)*cosa-cos(a+b)sina=0;………………（*）只要cosa与cos(a+b)不为0,由（*）式可得结论....