利用2倍角公式cosπ/9*cos2π/9*cos4π/9=[sinπ/9*cosπ/9*cos2π/9*cos4π/9]/sinπ/9=[1/2sin2π/9*cos2π/9*cos4π/9]/sinπ/9=[1/4sin4π/9*cos4π/9]/sinπ/9=[1/8sin8π/9]/sinπ/9=[1/8sinπ/9]/[sinπ/9]=1...