求值cosπ7cos2π7cos4π7= _ ．
问题描述：
求值cos
cosπ 7
cos2π 7
= ___ ．4π 7
答
原式=
=2sin
cosπ 7
cosπ 7
cos2π 7
4π 7 2sin
π 7
×1 2
=2sin
cos2π 7
cos2π 7
4π 7 2sin
π 7
=2sin
cos4π 7
4π 7 8sin
π 7
=sin
8π 7 8sin
π 7
=--sin
π 7 8sin
π 7
．1 8
故答案为：-
．1 8