单调递增：
2kπ-π≤2x-π/3≤2kπ
2kπ-2π/3≤2x≤2kπ+π/3
kπ-π/3≤x≤kπ+π/6；k∈Z
单调递减：
2kπ≤2x-π/3≤2kπ+π
2kπ+π/3≤2x≤2kπ+4π/3
kπ+π/6≤x≤kπ+2π/3 ；k∈Z