y=-(sinx-cosx)
=-√2sin(x-π/4)
则现在是y=-√2sin(x+m-π/4)
y轴即x=0
对称则此时sin取最值
sin(0+m-π/4)=±1
m-π/4=kπ+π/2
m=kπ+3π/4
所以k=0
m最小是3π/4