# Summation problem

1. Jan 6, 2014

### lionely

1. The problem statement, all variables and given/known data
Find the sum to n terms of the series whose rth term is 3r - 2r

2. Relevant equations

3. The attempt at a solution

So I tried this

rlog3 - rlog2 = n(n+1)/2 log3 - n(n+1)/2 log2

then I realized this was kind of useless, the only thing I could get from this is

n(n+1)/2 log (3/2) , then I don't know what to do with it.

the answer in the book is 3n+1/2 -2n+1 + 1/2

2. Jan 6, 2014

### Staff: Mentor

You have a mistake right off the bat. log(A - B) ≠ logA - logB.
Split your series into two - each part is a geometric series.

3. Jan 6, 2014

### lionely

I see it now thanks.. I keep making those darned mistakes with logs!!

4. Jan 6, 2014

### Staff: Mentor

It would be worth your while to review the properties of logs. They might be listed in an appendix of the book you're using. If not, get a precalc book or put in some time in an online tutorial such as khanacademy or the like.