sinx的减区间是(2kπ+π/2,2kπ+3π/2)
所以这里2kπ+π/2<2x+π/4<2kπ+3π/2
2kπ+3π/4<2x<2kπ+7π/4
kπ+3π/8所以减区间是(kπ+3π/8,kπ+7π/8)