cosπ/7+cos3π/7+cos5π/7
=(sinπ/7cosπ/7+sinπ/7cos3π/7+sinπ/7cos5π/7)/sinπ/7
=[sin2π/7+(sin4π/7-sin2π/7)+(sin6π/7-sin4π/7)]/(2sinπ/7) 括号里用积化和差
=sin6π/7/(2sinπ/7)
=sin(π-π/7)/(2sinπ/7)
=sinπ/7/(2sinπ/7)
=1/2
cosπ/7+cos3π/7+cos5π/7
=cosπ/7+cos3π/7-cos(π-5π/7) (利用公式 cos（A）=-cos（π-A）)
=cosπ/7+cos3π/7-cos(3π/7)
=cosπ/7
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