单增区间2x+π/3∈[2kπ-π/2, 2kπ+π/2]
x∈[kπ-5π/12, kπ+π/12] k∈Z
单减区间2k+π/3∈[2kπ+π/2, 2kπ+3π/2]
x∈[kπ+π/12, kπ+7π/12] k∈Z