求log以2为底cosπ/9的对数+log以2为底cos2π/9+log以2为底4π/9=

同底的对数相加,结果等于真数积的对数真数之积为cosπ/9cos2π/9cos4π/9=(8sinπ/9cosπ/9cos2π/9cos4π/9)/(8sinπ/9)=4sin2π/9cos2π/9cos4π/9)/(8sinπ/9)=(2sin4π/9cos4π/9)/(8sinπ/9)=(sin8π/9)/(8sinπ/9)=(sin(π-π/9))/(8sinπ/9)=1/8所以log2 (1/8)=-3,即原式=-3