y=3x^2+3x+1/x^2+x-1的值域
y=3x^2+3x-3+4/x^2+x-1
=3+4/x^2+x-1
(y-3)(x^2+x-1)=4
(y-3)x^2+(y-3)x+3-y-4=0
(y-3)x^2+(y-3)x-(y+1)=0
(y-3)^2+4(y-3)(y+1)>=0
(y-3)(y-3+4(y+1))>=0
(y-3)(5y+1)>=0
y=3