-π/6 ≤ x ≤π/6
-π/3 ≤ 2x ≤π/3
0 ≤2x+π/3 ≤ π/2
0 ≤ 3sin（2x+π/3 ）≤3
当x=0时,取得最小值为 0
当x=π/2时,取得最大值为 3