cos是偶函数
所以y=cos(2x-π/3)
cosx减区间是(2kπ,2kπ+π)
所以2kπ<2x-π/3<2kπ+π
2kπ+π/3<2x<2kπ+4π/3
kπ+π/6所以是(kπ+π/6,kπ+2π/3)