y+yx²=1+x+x²
(y-1)x²-x+(y-1)=0
判别式大于等于0
1-4(y-1)²≥0
(y-1)²≤1/4
-1/2≤y-1≤1/2
1/2≤y≤3/2
值域[1/2,3/2]