y=x^2-4x+5=(x-2)^2+1
当m≥2时,4^2+1≥y≥（m-2）^2+1=m^2-4m+5,即17≥y≥m^2-4m+5
当-2≤m<2时,4^2+1≥y≥1,即17≥y≥1
当m<-2时,(m-2)^2+1>y≥1,即m^2-4m+5>y≥1