运用向量方法求值；cos(2π/7)+cos(4π/7)+cos(6π/7)帮忙，详解。
热心网友
cos(2π/7)+cos(4π/7)+cos(6π/7)+cos(8π/7)+cos(10π/7)+cos(12π/7)+cos(14π/7)=0(七个大小一样方向间隔相同的力，合力为零）cos(2π/7)+cos(4π/7)+cos(6π/7)+cos(8π/7)+cos(10π/7)+cos(12π/7)=-cos(14π/7)=-1cos(2π/7)=cos(12π/7)cos(4π/7)=cos(10π/7)cos(6π/7)=cos(8π/7)cos(2π/7)+cos(4π/7)+cos(6π/7)=-0.5
热心网友
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