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A geometric sequence within a arithmetic sequence

  1. Aug 11, 2013 #1
    the main question here is that can a sequence * arithmetic * be correct if the difference is also changing in terms of a geometric sequence ?\
    now look at this sequence
    0.33,0.3333,0.333333
    now if we calculate the difference between the first two terms
    its 0.0033
    between the second and the third its
    0.000033
    the difference between the numbers goes as a geometric sequence
    0.33,0.0033,0.000033 and the Ratio between them is 1/100
    can this work as a sequence ? and if so what kind of sequence is it ?
     
  2. jcsd
  3. Aug 11, 2013 #2

    Stephen Tashi

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    Science Advisor

    What do you mean by "correct"?
     
  4. Aug 11, 2013 #3
    There are many different types of sequences. Some of the most common ones that we study are arithmetic and geometric sequences. Arithmetic sequences have a common difference while geometric sequences have a common ratio. If a sequence does not have a common difference or a common ratio then it is neither arithmetic or geometric but it is still a sequence.

    For example:
    1,2,3,4,5,6,.... is arithmetic but not geometric
    1,2,4,8,16,..... is geometric but not arithmetic
    1,4,9,16,25,.... is neither arithmetic or geometric but is still a sequence

    The sequence you mentioned is not arithmetic because it does not have a common difference. It is not geometric because there is no common ratio between terms. Actually, what you have is a sequence of partial sums of a geometric series.

    For example:
    S1 = 0.33
    S2 = 0.33 + 0.0033
    S3 = 0.33 + 0.0033 + 0.000033
    etc.
    So each partial sum individually is a geometric series but the sequence of partial sums is not arithmetic or geometric.

    Hope that makes sense,
    Junaid Mansuri
     
  5. Aug 11, 2013 #4
    right that makes sense , i was thinking of this because my teacher told me once about how to make a sequence of a 3.3333333and so on , i didn't think of it as getting the sum at the end but rather each term would be 0.33 ,0.3333 and so on
    so 0.333333333~ is actually a geometric sequence
    where it goes like 0.3,0.03,0.003 and then in the end if we take the sum of the series it gives 0.3333333
     
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