cos2π/7+cos4π/7+6π/7=

cos2π/7+cos4π/7+6π/7=
cos2π/7+cos4π/7+6π/7=

cos2π/7+cos4π/7+6π/7=
原式=(sinπ/7cos2π/7+sinπ/7cos4π/7+sinπ/7cos6π/7)/sinπ/7
分子=(sin3π/7-sinπ/7)/2+(sin5π/7-sin3π/7)/2+(sinπ-cos5π/7)/2
=-(sinπ/7)/2
所以原式=-1/2