For , the asymptote should in fact be .

To find out why, let's first examine your standard exponential graph

Ifais positive, then any power ofacan never make it negative. Even if you have a negative power, this will just give you a smaller and smaller fraction.

e.g

etc.

This is why can never be negative.

With , the asymptote has simply been translated down, since now can go 1 lower than it used to.