Hi

The following questions i am having problem getting the correct answers. I need help where i have gone wrong.

1)

Answers says its:

2)

Answers says its:

3)Find the second derivative of

The first derivative is

The second derivative i got:

Answers says its

4)If , show that

First derivative i got:

Now second derivative i got:

What did i do wrong? or what is the next step?

P.S