cosx的单调减区间：2kπ≤x≤2kπ+π
所以cos(2x+2π/3)+1的单调减区间：2kπ≤2x+2π/3≤2kπ+π
解得 2kπ-2π/3≤2x≤2kπ+π -2π/3
2kπ-2π/3≤2x≤2kπ+π/3
kπ-π/3≤x≤kπ+π/6
所以f(x)的单调减区间 [kπ-π/3,kπ+π/6]