y=(x^2+4x+5)/(x+1)
y(x+1)=(x^2+4x+5)
x^2+(4-y)x+5-y=0
△=(4-y)^2-4(5-y)=y^2-4y-4≥0
y≥2+2√2,或,y≤2-2√2
值域：(-∞,2-2√2]U[2+2√2,+∞)