Negative logarithm while finding limiting difference

Hi Guys,

I hit a dead end while solving this problem. I haven't done Complex numbers so far, so I am not sure how to proceed.

**By using logarithms, find approximately the difference between and where,**

I used the difference formula,

and substituting for a, r and n, I got,

But for the -0.7 I could have used the logarithm identities to solve this. But I can't take . The required answer is

How do I proceed? Thanks again for your help!

Edit:

I went back and calculated and individually, They are both identical . Hence the difference would be 0. Is the textbook answer wrong?