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
化简：SinA+2Sin3A+Sin5A/Sin3A+2Sin5A+Sin7A
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