令x-π/8=π 得x＝9π/8>2π/3.所以在已知的区间内取不到最小值－1
当x＝π/6时,cos(π/6-π/8)＝cosπ/24
当x＝2π/3时 cos（2π/3-π/8）=cos13π/24
cos13π/24