cosπ/9 cos2π/ 9cos3π/9 cos4π/9.=(sinπ/9cosπ/9 cos2π/ 9cos3π/9 cos4π/9.)/sinπ/9=(1/2sin2π/9cos2π/ 9cos3π/9 cos4π/9)/sinπ/9=(1/8sin8π/9cos3π/9)/sinπ/9=1/8*cos3π/9=1/16
你直接算不就得了