f(x) = (ax - 1)/(x + 1)
设 -1 < s < t < 0,则
(at - 1)/(t + 1) - (as - 1)/(s + 1) > 0
(at - 1)(s + 1) > (as - 1)(t + 1)
a(t - s) > -(t - s)
a > -1
同理,当0 < s < t < +∞时,求得a > -1。
所以a的取值范围是a∈(-1,+∞)
题目有问题吧,0不在定义域内
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