The domain of the function is , with .

Let

then

which implies .

Therefore its maximum value is .

On the other hand, attains its inferior value as (as you have shown).

For the second one, let

for (I believe it must be given on the positive halfline).

Clearly, as or , we have (superior value).

And on the other hand, we have

which implies

and thus for all (minimum value).

You may similarly show the third one too, I have to say that this one is very interesting, just show that its maximum values decrease and similarly show that its minimum values increase.