2cos9π/13cosπ/13+cos5π/13+cos3π/13(和差化积)=2cos9π/13cosπ/13+2 cos[(5π/13+3π/13)/2] cos[(5π/13-3π/13)/2]=2cosπ/13(cos9π/13+cos4π/13)=2cosπ/13[cos(π-4π/13)+cos4π/13]=2cosπ/13[-cos4π...