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Math Help - sequences

  1. #1
    Junior Member
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    sequences

    Hi, sorry can't seem to figure this out..please help me ..?

    1+3=4
    1+3+9=13
    1+3+9 +27 = 40
    and so on..

    find an expression for this sequence

    any guidance would help soo much .. so lost!!
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  2. #2
    MHF Contributor

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    Quote Originally Posted by Dgphru View Post
    Hi, sorry can't seem to figure this out..please help me ..?

    1+3=4
    1+3+9=13
    1+3+9 +27 = 40
    and so on..

    find an expression for this sequence

    any guidance would help soo much .. so lost!!
    Those are powers of three so this is a geometric series. The sum of the geometric series \sum_{i= 0}^n ar^{n}= a\frac{r^{n+1}- 1}{r- 1}. Here, a= 1, r= 3.
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