cosπ/9cos2π/9cos23π/9=[2sinπ/9cosπ/9cos2π/9cos23π/9]/2sinπ/9=[sin2π/9cos2π/9cos(2π+5π/9)]/2sinπ/9=[sin4π/9cos5π/9]/4sinπ/9=-[sin4π/9cos4π/9]/4sinπ/9=-(sin8π/9)/8sinπ/9=-(sinπ/9)/...