y=(2x^2+4x-7)/(x^2+2x+3)y=(2x^2+4x+6-13)/(x^2+2x+3)y=2-13/(x^2+2x+3)x^2+2x+3∈[2,+∞)则y=2-13/(x^2+2x+3)∈[-9/2,2)
变成y=2-13/(x^2+2x+3)由于(x^2+2x+3)=(x+1)^2+2>=2所以2-13/2=[-9/2,2]
