答案是0啊，从前面开始每两项加起来得0
cosπ/7+cos2π/7+cos3π/7+cos4π/7+cos5π/7+cos6π/7=(cosπ/7+cos2π/7+cos3π/7+cos4π/7+cos5π/7+cos6π/7)*cosπ/7/cosπ/7=(cosπ/7cosπ/7+cos2π/7cosπ/7+cos3π/7cosπ/7+cos4π/7cosπ/7+cos5π/7cosπ/7+cos6π/7cosπ/7)/cosπ/7=(1-cos2π/7+cosπ/7-cos3π/7+cos2π/7-cos4π/7+cos3π/7-cos5π/7+cos4π/7-cos6π/7+cos5π/7-cosπ)/(2cosπ/7)=(1+cosπ/7-cos6π/7-cosπ)/(2cosπ/7)=(2+2cosπ/7)/(2cosπ/7)=1/cosπ/7 +1
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