2x+π/4=π/2+2kπ x=π/8+kπ
2x+π/4=3π/2+2kπ x=7π/8+kπ
函数y=3sin(2x+π/4),x∈〔0,π〕的单调递减区间
[π/8+kπ,7π/8+kπ],k为整数