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Math Help - geo sequence

  1. #1
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    geo sequence

    1, 2, 4, 8, 16

    In the above sequence, the first term is 1 and each term after the first is twice the preceding term. What is the sum of the 16th, 17th, and 18th terms?

    I know 2^15 + 2^16 + 2^17 is the sum but how can I express it as 7(2^15)?
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  2. #2
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    Re: geo sequence

    Quote Originally Posted by dannyc View Post
    1, 2, 4, 8, 16

    In the above sequence, the first term is 1 and each term after the first is twice the preceding term. What is the sum of the 16th, 17th, and 18th terms?

    I know 2^15 + 2^16 + 2^17 is the sum but how can I express it as 7(2^15)?
    2^{15} + 2^{16} + 2^{17} = 2^{15}+2^1 \cdot 2^{15} + 2^2 \cdot 2^{15}

    Factor out 2^{15}.
    Thanks from dannyc
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