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GRE Prep Club Legend  Joined: 07 Jun 2014
Posts: 4857
GRE 1: Q167 V156 WE: Business Development (Energy and Utilities)
Followers: 105

Kudos [?]: 1776 , given: 397

The sequence S is defined by Sn = 2(Sn – 1) – 4 for each int [#permalink]
Expert's post 00:00

Question Stats: 83% (00:38) correct 16% (03:26) wrong based on 6 sessions
The sequence S is defined by $$S_n = 2(S_{n – 1}) – 4$$ for each integer n ≥ 2. If $$S_1 = 6$$, what is the value of $$S_5$$?

(A) –20
(B) 16
(C) 20
(D) 24
(E) 36
[Reveal] Spoiler: OA

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Sandy
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GRE Prep Club Legend  Joined: 07 Jun 2014
Posts: 4857
GRE 1: Q167 V156 WE: Business Development (Energy and Utilities)
Followers: 105

Kudos [?]: 1776 , given: 397

Re: The sequence S is defined by Sn = 2(Sn – 1) – 4 for each int [#permalink]
Expert's post
Explanation

The sequence $$S_n = 2(S_{n - 1}) - 4$$ can be read as “to get any term in sequence S, double the previous term and subtract 4.”

The problem gives $$S_1$$ (the first term) and asks for $$S_5$$ (the fifth term):

 6 $$S_1$$ $$S_2$$ $$S_3$$ $$S_4$$ $$S_5$$

To get any term, double the previous term and subtract 4. To get $$S_2$$, double $$S_1$$ (which is 6) and subtract 4: $$S_2 = 2(6) - 4 = 8$$.

Continue doubling each term and subtracting 4 to get the subsequent term:

 6 8 12 20 36 $$S_1$$ $$S_2$$ $$S_3$$ $$S_4$$ $$S_5$$

_________________

Sandy
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Try our free Online GRE Test Re: The sequence S is defined by Sn = 2(Sn – 1) – 4 for each int   [#permalink] 11 Aug 2018, 16:51
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