问题标题：
【COS(PAI/5)COS(2PAI/5)=?XIE】
问题描述：
COS(PAI/5)COS(2PAI/5)=?
XIE
谭颖回答：
=(2sinπ/5cosπ/5cos2π/5)/2sinπ/5
=(sin2π/5cos2π/5)/2sinπ/5
=(2sin2π/5cos2π/5)/4sinπ/5
=sin4π/5/4sinπ/5
=sin(π-π/5)/4sinπ/5
=sinπ/5/4sinπ/5
=1/4
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