由sin2a=2sina*cosa 和sin（a+b）=sinacosb+sinbcosa可得
sin3a=sin（2a+a）=sin2acosa+cos2asina=2sina*cosa cosa+cos2asina
所以sin3a/sina=2cosacosa+cos2a
由cos（a+b）=cosacosb-sinasinb可得
cos3a=cos（2a+a）=cos2acosa-sin2asina=cos2acosa-2cosasinasina
所以cos3a/cosa=cos2a-2sinasina
所以sin3a/sina-cos3a/cosa=2cosacosa+cos2a-cos2a+2sinasina
=2cosacosa++2sinasina
=2
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