cosπ/7cos2π/7cos3π/7
=2sinπ/7cosπ/7cos2π/7cos3π/7/2sinπ/7
=sin2π/7cos2π/7[-cos(π-3π/7)]/2sinπ/7
=-sin4π/7cos4π/7/4sinπ/7
=-sin8π/7/8sinπ/7
=sinπ/7/8sinπ/7
=1/8
添项
cosπ/7cos2π/7cos3π/7=cosπ/7sinπ/7cos2π/7cos3π/7/sinπ/7=
1/2sin2π/7....................时间不够了，明天再给你回答
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