原式=(4sinπ/5*cosπ/5*cos2π/5)/(4sinπ/5)
=(2sin2π/5*cos2π/5)/(4sinπ/5)
=(sin4π/5)/(4sinπ/5)
=1/4能够详细点吗？第一步我就看不懂了分子分母都乘以一个4sinπ/5那，最后一步呢分子sin4π/5=sin(π-π/5)=sinπ/5然后再分子分母约掉