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Math Help - Find the sum of the first 20 terms of this geometric sequence

  1. #1
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    Find the sum of the first 20 terms of this geometric sequence

    3,6,9,12

    can u explain plzz
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  2. #2
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    Hello, NeedHelp18!

    Find the sum of the first 20 terms of this geometric sequence:
    . . 3, 6, 9, 12, . . .

    can u explain plzz

    This will take a lot of explaining because . . .

    (1) You seem to know nothing about sequences . . .
    . . .especially the sum-formulas.

    (2) It is not a geometric sequence.

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  3. #3
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    is this right?

    Sn = (n/2)(a1 + an)
    S20 =(10)(3 + 60)
    = 630
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  4. #4
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    Quote Originally Posted by NeedHelp18 View Post
    is this right?

    Sn = (n/2)(a1 + an)
    S20 =(10)(3 + 60)
    = 630
    Yes, but note that this is an arithmetic sequence, not geometric.
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