Summation problem

  • Thread starter lionely
  • Start date
  • #1
576
2

Homework Statement


Find the sum to n terms of the series whose rth term is 3r - 2r




Homework Equations





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
 

Answers and Replies

  • #2
35,028
6,774

Homework Statement


Find the sum to n terms of the series whose rth term is 3r - 2r




Homework Equations





The Attempt at a Solution



So I tried this

rlog3 - rlog2 = n(n+1)/2 log3 - n(n+1)/2 log2
You have a mistake right off the bat. log(A - B) ≠ logA - logB.
Split your series into two - each part is a geometric series.
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
 
  • #3
576
2
I see it now thanks.. I keep making those darned mistakes with logs!!
 
  • #4
35,028
6,774
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.
 

Related Threads on Summation problem

  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
6
Views
3K
  • Last Post
Replies
2
Views
643
Replies
3
Views
2K
Replies
2
Views
2K
  • Last Post
Replies
8
Views
3K
Replies
4
Views
2K
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
7
Views
1K
  • Last Post
Replies
4
Views
2K
Top