y=3sin（2x+4分之派）
最小正周期为 2π/2=π
单调减区间：
2x+π/4∈[2kπ+π/2,2kπ+3π/2]
x∈[kπ+π/8,kπ+5π/8]
所以
单调减区间为;
[kπ+π/8,kπ+5π/8] k∈z