x∈(0,2π )
x+π/3∈(π/3,7π/3)
cosx的减区间是(2kπ,2kπ+π)
所以x+π/3∈(π/3,7π/3)时(π/3,π)和(2π,7π/3)为减区间
x∈(0,2π/3)和x∈(5π/3,2π)为减区间