sinb=sin[(a+b)-a]
=sin(a+b)cosa-cos(a+b)sina
=sin(a+b)cosa-sinb/sina*sina
=sin(a+b)cosa-sinb
2sinb=sin(a+b)cosa
因为：sinb/sina=cos(a+b),
所以,sinb=sinacos(a+b),
3sinb=sin(a+b)cosa+sinb
=sin(a+b)cosa+sinacos(a+b)
=sin(2a+b)
sinb/sina=cos(a+b),证明3sinb=sin(2a+b)
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