y=(x²-1)/(x²+1) = (x²+1-2)/(x²+1) = 1 - 2/(x²+1)
x²+1≥1
0＜2/(x²+1)≤2
-2≤-2/(x²+1)＜0
-1≤1-2/(x²+1)＜1
值域【-1，1）