y=cos(π/6)cos(2x)+sin(π/6)sin(2x)
=cos(2x-π/6)
2x-π/6在[2kπ,2kπ+π]单调递减
x在[kπ+π/12,kπ+7π/12]单调递减y=cos(π/6)cos(2x)+sin(π/6)sin(2x)=cos(π/6－2x)不行吗？cos(2x-π/6)=cos(π/6－2x)