判别式法.
x^2+2x+2=xy+y
x^2+(2-y)x+2-y=0
令△>=0
(2-y)^2-4(2-y)>=0
(2-y)(-y-2)>=0
(y-2)(y+2)>=0
所以值域负无穷到2,2到正无穷