求值!log2(cos湃/9)+log2(cos2湃/9)+log2(cos4湃/9)答案是-3,『湃就是3.1415926...』
问题描述：
求值!log2(cos湃/9)+log2(cos2湃/9)+log2(cos4湃/9)答案是-3,『湃就是3.1415926...』
答
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)