问题标题:
求证:tan(x+y)+tan(x-y)=sin2xcos2x−sin2y.
问题描述:

求证:tan(x+y)+tan(x-y)=sin2xcos2x−sin2y.

齐鲲鹏回答:
  证明:左=sin(x+y)cos(x+y)+sin(x−y)cos(x−y)
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