令z=2x+π/6 则y=3sinz
sinz 函数在 [ -π/2+2kπ,π/2+2kπ ]区间单调递增；
[ π/2+2kπ,3π/2+2kπ ]区间单调递减 k=0,±1,±2,±3……
当 -π/2+2kπ ≤ z ≤ π/2+2kπ 单调递增
-π/2+2kπ ≤ 2x+π/6 ≤ π/2+2kπ
所以 -π/3+kπ ≤ x ≤ π/6+kπ
[-π/3+kπ ,π/6+kπ] 单调递增
当 π/2+2kπ≤ z ≤ 3π/2+2kπ 单调递减
π/2+2kπ≤ 2x+π/6 ≤ 3π/2+2kπ
所以 π/6+kπ ≤ x ≤ 2π/3+kπ
[ π/6+kπ,2π/3+kπ] 单调递减