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DS - Cannot figure this out!

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Senior Manager
Joined: 26 Jul 2009
Posts: 317
DS - Cannot figure this out! [#permalink]

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26 Jul 2009, 08:10
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Need advice pls help!

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Manager
Joined: 10 Jul 2009
Posts: 149
Re: DS - Cannot figure this out! [#permalink]

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26 Jul 2009, 08:40
from 1,
xi = x(i-1)/2
we cannot arrive at solution
from 2
x5= x4/(x4 +1)
but we are unsure whether the same sequence is followed for the other numbers in the series.

combining 1 and 2
x5 = x4/2
x5= x4/(x4 +1)
solving we get x4 = 0 or 1
if x4 = 0 then all the terms will be zero, so x4 =1.
x4 = 1, x3 = 2, x2=4, x1 =8
So answer is "C"
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Re: DS - Cannot figure this out! [#permalink]

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26 Jul 2009, 13:00
Aleehsgonji wrote:
from 1,
xi = x(i-1)/2
we cannot arrive at solution
from 2
x5= x4/(x4 +1)
but we are unsure whether the same sequence is followed for the other numbers in the series.

combining 1 and 2
x5 = x4/2
x5= x4/(x4 +1)
solving we get x4 = 0 or 1
if x4 = 0 then all the terms will be zero, so x4 =1.
x4 = 1, x3 = 2, x2=4, x1 =8
So answer is "C"

Based on what you said, answer should be E....
we can have two value for X1 (0 or 8) so both are not sufficient.
Manager
Joined: 10 Jul 2009
Posts: 149
Re: DS - Cannot figure this out! [#permalink]

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26 Jul 2009, 13:37
For the zero solution then all the values in the sequence will be zero. If all the values are zero, should we still treat that as solution? Please confirm. Please post OA.
Senior Manager
Joined: 25 Jun 2009
Posts: 287
Re: DS - Cannot figure this out! [#permalink]

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27 Jul 2009, 01:16
Aleehsgonji wrote:
from 1,
xi = x(i-1)/2
we cannot arrive at solution
from 2
x5= x4/(x4 +1)
but we are unsure whether the same sequence is followed for the other numbers in the series.

combining 1 and 2
x5 = x4/2
x5= x4/(x4 +1)
solving we get x4 = 0 or 1
if x4 = 0 then all the terms will be zero, so x4 =1.
x4 = 1, x3 = 2, x2=4, x1 =8
So answer is "C"

Can you explain how did u get x4 =0 because when we solve these equations we get x4/2= x4/ x4 +1, and now if we simply cancel x4 in numerator then we get x4+1 = 2 , so now My question here is in equations like these shall we assume that answer can be 0 by looking at the initial equation or shall we simplify the equation and then look at the solution. ?

Thanks
Intern
Joined: 23 Jun 2009
Posts: 47
Re: DS - Cannot figure this out! [#permalink]

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27 Jul 2009, 02:11
Taking statement 1: It's obvious that we cannot determine X1 from that; however, we can deduce the following:
X2 = X1/2
X3 = X2/2
X4 = X3/2
etc.

Taking statement 2: There is nothing there that says X1 so there is no way we can get X1 from it alone.

Combining both statements: From the sequence in statement 1, we can tell that X5 = X4/2. Now, since we have another definition for X5 in statement 2 [i.e. X5 = X4/(X4+1)], we can equate both defnitions of X5 as follows:

X4/2 = X4/(X4+1)

Since the numerators are the same, we can then conclude from the denominators that X4+1 = 2, and therefore X4 = 1.

Take this information back to the sequence from statement 1, and you can find X3=3, then X2=4, and then X1=8.

Answer is C; you can solve with both statements, but not with any statement independetly.

Hope this helps.
Manager
Joined: 07 Apr 2009
Posts: 144
Re: DS - Cannot figure this out! [#permalink]

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27 Jul 2009, 13:00
Given its a sequence of positive numbers, wonder if we should even consider 0 , hence i would exclude x1 as 0.
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Posts: 261
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GPA: 3.42
WE: Engineering (Computer Hardware)
Re: DS - Cannot figure this out! [#permalink]

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27 Jul 2009, 17:01
C, you need both to derive X1, and 0 is not a positive number...
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Joined: 05 Jun 2009
Posts: 683
WE 1: 7years (Financial Services - Consultant, BA)
Re: DS - Cannot figure this out! [#permalink]

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28 Jul 2009, 00:00
skpMatcha wrote:
Given its a sequence of positive numbers, wonder if we should even consider 0 , hence i would exclude x1 as 0.

agree,

Sequence is of positive numbers (Xi<>0), Hence C.
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Senior Manager
Joined: 26 Jul 2009
Posts: 317
Re: DS - Cannot figure this out! [#permalink]

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30 Jul 2009, 19:30
OA is 'C'. Thanks all.

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: DS - Cannot figure this out!   [#permalink] 30 Jul 2009, 19:30
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