# A simple ln differentiation question

• Oct 26th 2009, 04:13 AM
davemk
A simple ln differentiation question
Hi

I'm somewhat embarassed to be asking such a simple question but hopefully someone would be kind enough to help me out.

I've been asked to find the first and second derivatives of A = ln x so I got the following, which agreed with the answers given:

A' = 1/x

However, my confusion is with the second derivative. I prefer to use x^-1 rather than 1/x. Using x^-1 would give me -1*x^-1-1:

A'' = -x^-2 or -1/x^2

The answer given is 1/x^2 (ie positive, rather than the negative answer I get).

Would somebody be kind enough to tell me which is correct and, if I'm wrong, please explain where I've gone wrong. Thanks very much.

Dave.
• Oct 26th 2009, 04:25 AM
Gusbob
You are correct.
• Oct 26th 2009, 04:30 AM
davemk
Thank you.

I assumed that the lecturers answers would be correct so I've been trying to work out why my answer is wrong for ages. Oh well, at least I've got it right.

Thanks again, your help is much appreciated.

Dave.