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Math Help - Summation problem......

  1. #1
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    Summation problem......

    How do I find the summation of 1+3+9+........+n without assuming any summation formula?
    .
    .
    Thanks a lot in advance................
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  2. #2
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    As I can see from the sequence, I see that it is a geometric sequence.

    You can use the formula: S_n=\dfrac{ar^n - a}{r - 1}

    Where:
    a = the first term
    r = common ratio, which in this case is 3
    n = nth term that it goes to
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  3. #3
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    Quote Originally Posted by Educated View Post
    As I can see from the sequence, I see that it is a geometric sequence.

    You can use the formula: S_n=\dfrac{ar^n - a}{r - 1}

    Where:
    a = the first term
    r = common ratio, which in this case is 3
    n = nth term that it goes to
    But I said without assuming any summation formula ....
    Last edited by mr fantastic; October 20th 2010 at 01:13 AM.
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  4. #4
    Senior Member Educated's Avatar
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    Oh, I thought you meant without the sigma notation \sum.

    In that case, the only way that I can think of is by just adding them up manually.
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  5. #5
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    Quote Originally Posted by Arka View Post
    But I said without assuming any summation formula ....
    What you do is reproduce the working that leads to the general formula for this particular question, that is, derive the formula from first principles for this particular question.
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  6. #6
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    Thanks a lot..............Mr. Fantastic....
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