sinA sin5A=sin(3A-2A) sin(3A 2A) =2sin3Acos2A 同理有 sin3A sin7A=2sin5Acos2A 所以原式=（cos2A 1）sin3A/(cos2A 1)sin5A =sin3A/sin5A
求证:sin5a+sina=2sin3a*cos2a
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