若sinb=2sin(2a-b),(a≠丌/2+k丌,a-b≠丌/2+k丌,k属于Z),则tan(a-b)/tan a的值、 急-

设x=a-b
y=a
则b=y-x
2a-b=x+y
即sin(y-x)=2sin(x+y)
sinycosx-cosysinx=2sinxcosy+2cosxsiny
3sinxcosy=-cosxsiny
tanx/tany=-1/3
即tan(a-b)/tana=-1/3