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Number sequence

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Senior Manager
Senior Manager
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Joined: 21 Jul 2009
Posts: 266
Location: New York, NY
Followers: 1

Kudos [?]: 37 [0], given: 23

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Number sequence [#permalink] New post 24 Aug 2009, 17:22
00:00
A
B
C
D
E

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
[Reveal] Spoiler:
A

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Manager
Manager
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Joined: 10 Aug 2009
Posts: 130
Followers: 3

Kudos [?]: 54 [0], given: 10

Re: Number sequence [#permalink] New post 24 Aug 2009, 18:48
x_1=3

x_2=2\times3-1

x_3=2\times(2\times3-1)-1=2^2\times3-3

x_4=2\times(2^2\times3-3)-1=2^3\times3-7

testing the difference between:

x_3-x_2=2^2

x_4-x_3=2^3

there is a pattern : x_n-x_{n-1}=2^{n-1}

therefore, x_{20}-x_{19}=2^{19}
Manager
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Joined: 28 Jul 2009
Posts: 126
Location: India
Schools: NUS, NTU, SMU, AGSM, Melbourne School of Business
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Kudos [?]: 53 [0], given: 12

GMAT Tests User
Re: Number sequence [#permalink] New post 24 Aug 2009, 21:37
LenaA wrote:
x_1=3

x_2=2\times3-1

x_3=2\times(2\times3-1)-1=2^2\times3-3

x_4=2\times(2^2\times3-3)-1=2^3\times3-7

testing the difference between:

x_3-x_2=2^2

x_4-x_3=2^3

there is a pattern : x_n-x_{n-1}=2^{n-1}

therefore, x_{20}-x_{19}=2^{19}


Nice explaination.
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Re: Number sequence   [#permalink] 24 Aug 2009, 21:37
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