(a)

The limit is .

Rewrite the limit.

If tends to , then approaches to .

Therefore, .

(b)

The limit is .

Rewrite the limit.

If tends to   from the right side, approaches to .

Therefore, .

(c)

The limit is .

Rewrite the limit.

If tends to   from the right side, approaches to .

Therefore, .

(d)

The limit is .

Rewrite the limit.

If tends to then approaches to .

Therefore, .

(a).

(b) .

(c) .

(d) .