题目

cos9分之派cos9分之2派cos9分之4派等于多少?

提问时间：2020-11-03

答案

原式=2sin(π/9)cos(π/9)cos(2π/9)cos(4π/9)/2sin(π/9)
=sin(2π/9)cos(2π/9)cos(4π/9)/2sin(π/9)
=2sin(2π/9)cos(2π/9)cos(4π/9)/4sin(π/9)
=sin(4π/9)cos(4π/9)/4sin(π/9)
=sin(8π/9)/8sin(π/9)
=sin(π-8π/9)/8sin(π/9)
=sin(π/9)/8sin(π/9)
=1/8