要过程
定义域都是R
1.y=(x^2+1-2)/(x^2+1)=1-2/(x^2+1)
x^2+1>=1
0<1/(x^2+1)<=1
-2<=-2/(x^2+1)<0
-1<=y<1
2.和1差不多,取相反数,-1<y<=1