证明：因为tan(x+y)=2tany所以,sin(x+y)÷cos(x+y)=2siny÷cosy所以,sin(x+y)cosy=2cos(x+y)siny所以,2sin(x+y)cosy=4cos(x+y)siny所以,3sin(x+y)cosy-3cos(x+y)siny=sin(x+y)cosy+cos(x+y)siny所以,3sin[(x+y)-y]=s...