Because the domain x of f(x) is always greater than 0.

So when a is greater than or equal to 0, f ` (x) is always greater than 0, which is an interval.

But when a is less than-1, a+ 1 is less than 0, so the numerator of f ` (x) is always less than 0 and the denominator is greater than 0. At this time, f` (x) is always less than 0.

And in the interval of-1 < a < 0, it will be discussed separately.

This depends on observation!