1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Geometric Sequence Formula

  1. Jul 8, 2008 #1
    1. The problem statement, all variables and given/known data
    On an exam question, although I can not remember the details, it gave us a table that looks similar to the table below.
    http://img403.imageshack.us/img403/9703/geoseqpr4.png [Broken]​
    The question tells us that the graph is a geometric sequence, and says to use a formula to determine what the common ratio is.

    2. Relevant equations

    3. The attempt at a solution
    Although the question did not specify which formula to use, I used the second one that I listed above. Would this be acceptable to determine the common ratio, or was the question expecting us to use the general term formula?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jul 8, 2008 #2
    Both formulas can be used. (why?)
  4. Jul 8, 2008 #3


    User Avatar
    Science Advisor

    Those are both the same with A= tn, r= 1+ i, and a= P(1+i).

    However, the second equation is NOT normally given as a 'geometric sequence' formula. It is the formula for the amount in, say, a bank account, when the initial amount put into the account was P and the account compounded annually, for n years, at interest i.

    You should be able to get the correct answer using either one. did the problem really say to "use a formula"? Learning mathematics is more about learning definitions than formulas. I would simply say that the common ratio is simply the ratio of the population at one year to the population the previous year- if this is a geometric sequence then all such ratios should be the same: 20/10= 40/20= 2. The "common ratio" is 2.
  5. Jul 8, 2008 #4
    Actually, I think the question more specifically asked to use a geometric sequence formula, and I used the second one that I listed. Now, I'm wondering if I will lose marks for not using the general term formula, because it is known as the geometric sequence formula. The reason why I used the second formula on the exam is because I thought it was just another form of the general term formula, and that it would be acceptable.
  6. Jul 8, 2008 #5
    Like I said at first, I don't quite remember the exact details of the question, but you had to find the common ratio (although they don't tell you directly) and put it into a percentage. Also, if you read my post above, you will see that I corrected myself.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook