x>=0,则x+1>0
(x-1)/(x+1)-a=0,则x-1=a(x+1),(a-1)x=-—(a+1)
若a=1,上式为0=-2,等式不成立,故a≠1
x=-(a+1)/(a-1)
因为x≥0,所以-(a+1)/(a-1)≥0
即(a+1)(a-1)≤0
-1≤a≤1
又a≠1
故a取值范围为-1≤a＜1