# Thread: integral issue with ln

1. ## integral issue with ln

This is part of a larger mixing problem, but I'm confused with this particular part. I got to an integral of:

$\int{\frac{1}{30-A}dA}$ which to me is pretty straight-forward as just $ln|30-A|$. For some reason, the book says the answer is $-ln|30-A|$. Can someone explain where the negative came from?

I can post the full problem if that's important.

2. ## Re: integral issue with ln

you're integrating $\frac{u'}{u}$ ... what is the derivative of (30-A) ?

3. ## Re: integral issue with ln

oh.... ok, I got it, thanks.