函数y=lgsin(2x+π/2)的单调递减
0＜sin(2x+π/2)≤1递减
2kπ+π/2≤2x+π/2＜2kπ+π
2kπ≤2x＜2kπ+π/2
kπ≤x＜kπ+π/4