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if the term series is given is given and no condition is

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if the term series is given is given and no condition is [#permalink]

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New post 07 Aug 2008, 18:11
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if the term series is given is given and no condition is given, is it ok to assume that all the terms follow the same general formula?

what should be the ans here?
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Re: GMAT PREP SERIES... [#permalink]

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New post 07 Aug 2008, 18:34
arjtryarjtry wrote:
if the term series is given is given and no condition is given, is it ok to assume that all the terms follow the same general formula?

what should be the ans here?


(1) alone doesn't tell us what Xi could be. It just tells us that each subsequent is half of the previous one
e.g. Xi could be any number

(2) just gives us the relation between X4 and X5.

When we combine the two, we get

X5 = X4/2 = X4/X4+1

==> X4 + 1 = 2
==> X4 = 1

X5 = .5
X4 = 1
X3 = 2
X2 = 4
X1 = 8

Looks like C

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New post 07 Aug 2008, 18:55
but on combining the two, THE QUESTION NEVER TELLS US X1. it just tells ,x>1. so how do u get x1?

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Re: GMAT PREP SERIES... [#permalink]

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New post 07 Aug 2008, 20:13
arjtryarjtry wrote:
but on combining the two, THE QUESTION NEVER TELLS US X1. it just tells ,x>1. so how do u get x1?


From (1) X5 = X4/2
From (2) X5 = X4/X4+1

So, I equated denominators. And problem says formula in (1) valid for i>1, but question stem says sequence starts with X1. After finding X5, I went backwards to find X1. Whats the OA supposed to be?

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Re: GMAT PREP SERIES... [#permalink]

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New post 08 Aug 2008, 07:12
arjtryarjtry wrote:
if the term series is given is given and no condition is given, is it ok to assume that all the terms follow the same general formula?

what should be the ans here?



No You can't assume/generalize

1) Xi = Xi-1/2
--> Xi-1/2 = Xi
--> X1= X2*2
We don't the value of X2..
Insufficient
2) X5= X4/ (X4+1)
insufficient.. we can't generalize this formula.

combine
X5= X4/ (X4+1)
--> you can find the X
X5 = X4/2
Solve these two equations will get the value of X4
using X4 value and equation (1) you can find X3
similarly using X3 value and equation (1) you can find X2
similarly using X2 value and equation (1) you can find X1


together sufficient.
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Re: GMAT PREP SERIES... [#permalink]

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New post 08 Aug 2008, 07:14
arjtryarjtry wrote:
but on combining the two, THE QUESTION NEVER TELLS US X1. it just tells ,x>1. so how do u get x1?


combining the two, we get x4.
once we have x4, we can find x3 using statement 1, remember statement 1 is for any i > 1, so it'll work for i = 4,3,2
at i = 2, we'll can find x1

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Re: GMAT PREP SERIES...   [#permalink] 08 Aug 2008, 07:14
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