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Manager
Joined: 31 Oct 2007
Posts: 117
Location: Frankfurt, Germany
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If the sequence x{_1} , x{_2} , x{_3} , ...., x{_n} , ....is [#permalink]
 17 Jul 2008, 03:03
If the sequence x{_1}, x{_2}, x{_3}, ...., x{_n}, ....is such that x{_1} = 3 and x{_n_+_1} = 2x{_n} - 1 for n >= 1, then x{_2_0} - x{_1_9} = A. 2^{19}B. 2^{20}C. 2^{21}D. 2^{20} -1E. 2^{21} -1
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Jimmy Low, Frankfurt, Germany
Blog: http://mytrainmaster.wordpress.com
GMAT Malaysia: http://gmatmalaysia.blogspot.com
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Joined: 12 Jul 2008
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Re: Math Set 1: Q25 [#permalink]
17 Jul 2008, 03:51
1
This post received
KUDOS
jimmylow wrote: If the sequence x{_1}, x{_2}, x{_3}, ...., x{_n}, ....is such that x{_1} = 3 and x{_n_+_1} = 2x{_n} - 1 for n >= 1, then x{_2_0} - x{_1_9} =
A. 2^{19}
B. 2^{20}
C. 2^{21}
D. 2^{20} -1
E. 2^{21} -1 A if you do out the first few, you'll see a pattern. x1 = 3 x2 = 2*3-1 = 5 x3 = 2*5-1 = 9 x4 = 2*9-1 = 17 x2-x1 = 2 x3-x2 = 4 = 2^2 x4-x3 = 2^3 . . . x20-x19 = 2^19
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Manager
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Re: Math Set 1: Q25 [#permalink]
17 Jul 2008, 04:00
Thks mate. 1 kudos for you
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Jimmy Low, Frankfurt, Germany
Blog: http://mytrainmaster.wordpress.com
GMAT Malaysia: http://gmatmalaysia.blogspot.com
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Re: Math Set 1: Q25 [#permalink]
17 Jul 2008, 04:46
jimmylow wrote: If the sequence x{_1}, x{_2}, x{_3}, ...., x{_n}, ....is such that x{_1} = 3 and x{_n_+_1} = 2x{_n} - 1 for n >= 1, then x{_2_0} - x{_1_9} =
A. 2^{19}
B. 2^{20}
C. 2^{21}
D. 2^{20} -1
E. 2^{21} -1 every term in sequence is 2^n + 1....where n >=1 x20 = 2^20 + 1 x19 = 2^19 + 1 subtracting x19 from x20 2*2^19 + 1 - 2^19 -1 2^19(2-1) 2^19 A.
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Re: Math Set 1: Q25
[#permalink]
17 Jul 2008, 04:46
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