由π/2+2kπ≤2x+π/6≤3π/2+2kπ,k∈Z,得：
π/3+2kπ≤2x≤4π/3+2kπ,k∈Z,
两边同时除以2,得：π/6+kπ≤x≤2π/3+kπ,k∈Z,
故：所求的单调减区间是：[π/6+kπ,2π/3+kπ],k∈Z