令a=x+y,则条件变为
tan(x+y)=2tanx于是tana=2tanx,
2sinacosx=4cosasinx
3sinacosx-3cosasinx=sinacosx+cosasinx
3sin(a-x)=sin(a+x)
所以
3siny=sin（2x+y）
若tan（x+y）=2tanx,求证3siny=sin（2x+y）.
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