cosπ/5×cos2π/5=
问题描述:
cosπ/5×cos2π/5=
答
cosπ/5×cos2π/5=(2sinπ/5cosπ/5×cos2π/5)/2sinπ/5=(2sin2π/5×cos2π/5)/(4sinπ/5)
=(sin4π/5)/(4sinπ/5)=1/4