原式=2sinπ/5cosπ/5cosπ/5/(2sinπ/5)
=sin2π/5cosπ/5/(2sinπ/5)
=2sin2π/5cosπ/5/(4sinπ/5)
=sin4π/5/(4sinπ/5)
=sin(π-π/5)/(4sinπ/5)
=sinπ/5/(4sinπ/5)
=1/4
求COSπ/5*COS2π/5
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