y=3sin(2x+π/6)
x∈【0,π】
2x+π/6∈【π/6,2π+π/6】
当2x+π/6∈(π/2,3π/2)时单调减,此时x∈（π/3,2π/3)
故单调减区间（π/3,2π/3)