y=(x²+1)/(x²-1)
=1+2/(x²-1)
x²-1>=-1
函数值域(1,+∞)∪(-∞,-1]
y=(2x²+4x-7)/(x²+2x+1)
=[2(x+1)²-9]/(x+1)²
=2-9/(x+1)²
函数值域(-∞,2)