cos(2π/7)+cos(4π/7)+cos(6π/7)= -1/2[cos(2π/7)+cos(4π/7)+cos(6π/7)]^2= 1/4[cos(2π/7)]^2+[cos(4π/7)]^2+[cos(6π/7)]^2+2cos(2π/7)cos(4π/7)+2cos(2π/7)cos(6π/7)+2cos(4π/7)cos(6π/7)=1/4则[cos(...