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Math Help - Annuities and Geometric Series

  1. #1
    Junior Member VDestinV's Avatar
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    Exclamation Annuities and Geometric Series

    Okay so I understand how to do these questions when I have the last term. But I do not know how to find n when I only have the first three terms. Here's an example:

    S10 in the series $2500 + $2562.50 + %2626.56 + ...

    so am I still supposed to use the formula tn=ar^n-1?

    if so how do i find the tn?
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  2. #2
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    Quote Originally Posted by VDestinV View Post
    Okay so I understand how to do these questions when I have the last term. But I do not know how to find n when I only have the first three terms. Here's an example:

    S10 in the series $2500 + $2562.50 + %2626.56 + ...

    so am I still supposed to use the formula tn=ar^n-1?

    if so how do i find the tn?
    Well for S10, n would be 10, r would be (2562.2/2500) and a would be 2500.
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  3. #3
    Junior Member VDestinV's Avatar
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    Quote Originally Posted by sean.1986 View Post
    Well for S10, n would be 10, r would be (2562.2/2500) and a would be 2500.
    but like in this:
    tn= ar^n-1
    640 = 5(2)^n-1
    128 = (2)^n-1
    (2)^7=(2)n-1
    7=n-1
    n=8

    I wanna be able to do that but I can not because I do not know what tn equals.
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  4. #4
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    Isn't Tn just the nth term for an arbitrary n? Which term are you trying to find?
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  5. #5
    Junior Member VDestinV's Avatar
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    Quote Originally Posted by sean.1986 View Post
    Isn't Tn just the nth term for an arbitrary n? Which term are you trying to find?
    I'm actually trying to find the sum but first I have to find n, n is the term number whereas tn is the actual amount. I'm trying to find t i guess
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  6. #6
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    Why don't you use the formula for the sum?

    Sn = \frac{a(1 - r^n)}{1-r}
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  7. #7
    Junior Member VDestinV's Avatar
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    Quote Originally Posted by sean.1986 View Post
    Why don't you use the formula for the sum?

    Sn = \frac{a(1 - r^n)}{1-r}
    because I need to know n
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  8. #8
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    So you know the nth term but not what n is? What's the nth term?
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  9. #9
    Junior Member VDestinV's Avatar
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    Quote Originally Posted by sean.1986 View Post
    So you know the nth term but not what n is? What's the nth term?
    10 i think
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  10. #10
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    That would be n, not the nth term.

    If you're looking for the 10th term, then...

    r = \frac{2562.5}{2500}

    a10 = 2500r^9
    Last edited by sean.1986; June 8th 2008 at 03:38 PM.
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  11. #11
    Junior Member VDestinV's Avatar
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    look im sorry but you are confusing me.
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