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The sequence s1, s2, s3, ..., sn, ... is such that Sn= 1/n - 1/(n + 1)

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Wharton Thread Master
Joined: 30 May 2015
Posts: 40
Re: The sequence s1, s2, s3, ..., sn, ... is such that Sn= 1/n - 1/(n + 1)  [#permalink]

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16 Jul 2017, 09:39
Solve it by identifying the Pattern
S1 = 1/2
S2 = 1/6 => S1 +S2 = (1/2 + 1/6) = 2/3
S3 = 1/12 => ((S1+S2) + S3) = (2/3 + 1/12) = 3/4
S4 = 1/20 => ((S1+S2+S3) + S4) = (3/4 + 1/20) = 4/5
.
.
and so on

Option (a) => k>10
As per the pattern above sum until S9 = 9/10 and S10 = 10/11 , so k>10 will always be greater than 9/10
Sufficient

Option (b) => K<19
As per pattern above, clearly sum of k terms will be greater than 9/10 for 9<K<19 and lesser than or equal to 9/10 for 1=<K <=9 .
InSufficient
Manager
Joined: 24 Oct 2016
Posts: 76
GMAT 1: 670 Q46 V36
Re: The sequence s1, s2, s3, ..., sn, ... is such that Sn= 1/n - 1/(n + 1)  [#permalink]

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21 Jul 2018, 08:25
Bunuel gmatbusters

This question is a duplicate of this Q (https://gmatclub.com/forum/the-sequence ... ml?kudos=1) and hence, it should be moved to the archive forum. Otherwise, having both same questions in this forum would beat the purpose of the timer icon (green or red) that shows whether one's attempted a question before.
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Manager
Joined: 24 Oct 2016
Posts: 76
GMAT 1: 670 Q46 V36
Re: The sequence s1, s2, s3, ..., sn, ... is such that Sn= 1/n - 1/(n + 1)  [#permalink]

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21 Jul 2018, 08:27
Bunuel gmatbusters

This question is a duplicate of this Q (https://gmatclub.com/forum/the-sequence ... ml?kudos=1) and hence, it should be moved to the archive forum. Otherwise, having both same questions in this forum would beat the purpose of the timer icon (green or red) that shows whether one's attempted a question before.
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Re: The sequence s1, s2, s3, ..., sn, ... is such that Sn= 1/n - 1/(n + 1) &nbs [#permalink] 21 Jul 2018, 08:27

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