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]=sin[(x+y)+y]
所以,3sinx=sin(x+2y)
tan(x+y)=2tany,求证3sinx=sin(x+2y)
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