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Find the first three terms a geometric sequence

  1. Sep 15, 2010 #1
    1. The problem statement, all variables and given/known data

    Find the first three terms of a geometric sequence given that the sum of the first four terms is 65/3 and the sum to infinity is 27.

    2. Relevant equations

    S_n = \frac{a(1 - r^n)}{1 - r}\\
    S_n = \frac{a(r^n - 1}{r - 1}

    3. The attempt at a solution

    I'm trying to get two equations in two unknowns.

    I end with

    a + 27r = 27

    But I get lost after that. Trying to solve the sum leaves me with a equation to fourth or fifth power which I can't solve.

    I'm sure there's a simple approach to this.
    Last edited by a moderator: Sep 16, 2010
  2. jcsd
  3. Sep 15, 2010 #2


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    Gold Member

    Do you know the formula for the sum to infinity for a geometric series?


    EDIT: Sorry, just realised that is where you got your equation from.

    Consider this:

    {S_\infty } = \frac{a}{{1 - r}}\\
    {S_3} = \frac{{a(1 - {r^3})}}{{1 - r}} = {S_\infty }(1 - {r^3})

    Do you see how that can help?
    Last edited: Sep 15, 2010
  4. Sep 15, 2010 #3
    is there a latex form to be typed
  5. Sep 16, 2010 #4
    For some reason my latex code isn't working. Latex is voodoo magic. No doubt about it.
  6. Sep 16, 2010 #5
    I'm still stuck.

    So I end up with

    [tex] S_3 = 27(1 - r^3) [/tex]

    Then where do I go from there? Trying to find the common ratio leaves me with a mess. I know I should end up with a quadratic equation and two sets of solutions, but I completely stumped on how to get there.
  7. Sep 16, 2010 #6


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    Homework Helper

    Using the same idea Danago has given, what is S4?
  8. Sep 16, 2010 #7


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    Science Advisor

    Either that or you used "[\tex]" rather than the correct "[/tex]" to end it! I have corrected it.
  9. Sep 16, 2010 #8
    I got it! Solving for S_4 as above yields a value for r, and from there the other values can be solved, not a quadratic equation in sight.

    Thank you. I'm a bit slow on the uptake, but I'm beginning to understand how you guys work now. You don't just hand out the fish, you instead teach us how to fish. Your help is highly appreciated. Thanks again.
  10. Sep 16, 2010 #9


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    Homework Helper

    Good on you :smile:

    Well of course! Your markers are going to determine how well you can fish by showing them there and then what you can do, not what fish you've caught in your homework :tongue:

    I think I took the fish analogy too far hehe... Best of luck Notaphysicist!
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