3sinx=sin(2y+x)
3sin(x+y-y)=sin(y+x+y)
3[sin(x+y)cosy-cos(x+y)siny]=sin(x+y)cosy+cos(x+y)siny
sin(x+y)cosy=2cos(x+y)siny
sin(x+y)/cos(x+y)=2siny/cosy
tan(x+y)=2tany
已知3sinx=sin（2y＋x）,求证tan（x＋y）=2tany
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