原式=log2(cosπ/9cos2π/9cos4π/9)=log2(sinπ/9cosπ/9cos2π/9cos4π/9)/(sinπ/9)=log2(sin2π/9cos2π/9cos4π/9)/(2sinπ/9)=log2(sin4π/9cos4π/9)/(4sinπ/9)=log2(sin8π/9)/(8sinπ/9)