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The sequence a1, a2, … , a n, … is such that an = 2an-1 - x

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The sequence a1, a2, … , a n, … is such that an = 2an-1 - x [#permalink]

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04 Jun 2007, 20:23
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The sequence $$a_1$$, $$a_2$$, … , $$a_n$$, … is such that $$a_n = 2a_{n-1} - x$$ for all positive integers n ≥ 2 and for certain number x. If $$a_5 = 99$$ and $$a_3 = 27$$, what is the value of x?

A. 3
B. 9
C. 18
D. 36
E. 45
[Reveal] Spoiler: OA

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Last edited by Bunuel on 24 Dec 2013, 01:16, edited 1 time in total.
Renamed the topic, edited the question and added the OA.

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Re: PS - Sequence (a1, a2, …) [#permalink]

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05 Jun 2007, 02:37
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jet1445 wrote:
Q13:
The sequence a1, a2, … , a n, … is such that an = 2an-1 - x for all positive integers n ≥ 2 and for certain number x. If a5 = 99 and a3 = 27, what is the value of x?

A. 3
B. 9
C. 18
D. 36
E. 45

a5= 2*a4 - x = 99

a4 = 2*a3 - x = 2*27 - x

therefore;

a5 = 2*(54 - x ) -x = 99

108 - 3*x = 99

therefore X = 3

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28 Aug 2010, 07:30
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Good one. Please let me know if some one comes up with a solution that can be worked under 2 mins

given an= 2an-1 - x

a5 = 99 = 2 a4 - X = 2[2a3-X] -X = 4 a3 - 3X
given a3 = 27 ; substituting:

108 - 3X = 99 => X = 3

A

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28 Aug 2010, 10:13
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udaymathapati wrote:
The sequence a1, a2, … , a n, … is such that an = 2an-1 - x for all positive integers n ≥ 2 and for certain number x. If a5 = 99 and a3 = 27, what is the value of x?
A. 3
B. 9
C. 18
D. 36
E.45

Quite simple to solve this one.

Given: $$a_n = 2a_{n-1} - x$$

$$a_5 = 99$$

$$a_3 = 27$$

$$a_5 = 2a_4 - x = 2(2a_3 - x) - x = 4a_3 - 3x = 99$$

$$4(27) - 3x = 99$$$$3x = 108-99 = 9$$

$$x = 3$$

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20 Oct 2010, 06:25
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The sequence a1…..a2.......an........is such that an=2an-1-X for all positive integers n>=2 and for certain number X. If a5=99 and a3=27, what is the value of X?
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20 Oct 2010, 06:50
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monirjewel wrote:
The sequence a1…..a2.......an........is such that an=2an-1-X for all positive integers n>=2 and for certain number X. If a5=99 and a3=27, what is the value of X?

Plug the known values a5=99 and a3 = 27 into the formula:

a5 = 2(a4) - x
99 = 2(a4) - x

a4 = 2(a3) - x
a4 = 2(27)-x = 54-x

Substitute 54-x for a4 in the top equation:
99 = 2(54-x)-x
99=108-3x
3x=9
x=3

On the GMAT, I would recommend that you plug in the answer choices, one of which would say that x=3.

Plug a5 = 99 and x=3 into the formula:
99 = 2(a4) -3
a4 = 51

Plug a4=51, a3=27, and x=3 into the formula:
51 = 2(27) - 3
51 = 51. Success!
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20 Oct 2010, 06:56
monirjewel wrote:
The sequence a1…..a2.......an........is such that an=2an-1-X for all positive integers n>=2 and for certain number X. If a5=99 and a3=27, what is the value of X?

A5=99 and A3=27
According to the given nth term, A5=2(A4)-x=2{2(A3)-x}-x=2{(2*27)-x}-x=108-2x-x=108-3x
Hence 108-3x=99
or x=9/3=3

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Re: PS - Sequence (a1, a2, …) [#permalink]

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11 Sep 2011, 10:03
a(n) = 2*a(n-1) -x

a5 = 99
a3=27

a5 = 2a4-x
a4 = 2a3-x

=>99 = 2(2a3-x)-x

99 = 4a3-3x = 4*27-3x

=>x=3

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Re: PS - Sequence (a1, a2, …) [#permalink]

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12 Sep 2011, 05:14
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a4= 54-x

=> a5 = 2 (54 -x) -x = 99

=> 108 - 3x = 99
=> x= 3

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Re: Q13: The sequence a1, a2, , a n, is such that an = 2an-1 [#permalink]

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23 Dec 2013, 11:58
Hello from the GMAT Club BumpBot!

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Re: The sequence a1, a2, … , a n, … is such that an = 2an-1 - x [#permalink]

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04 Jul 2014, 08:51
whiplash2411 wrote:
udaymathapati wrote:
The sequence a1, a2, … , a n, … is such that an = 2an-1 - x for all positive integers n ≥ 2 and for certain number x. If a5 = 99 and a3 = 27, what is the value of x?
A. 3
B. 9
C. 18
D. 36
E.45

Quite simple to solve this one.

Given: $$a_n = 2a_{n-1} - x$$

$$a_5 = 99$$

$$a_3 = 27$$

$$a_5 = 2a_4 - x = 2(2a_3 - x) - x = 4a_3 - 3x = 99$$

$$4(27) - 3x = 99$$$$3x = 108-99 = 9$$

$$x = 3$$

QUESTION : How exactly did you get from 2(2a3 - x) to 4a3 * 3x, wouldn't it be 4a3 - 2x?

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Re: The sequence a1, a2, … , a n, … is such that an = 2an-1 - x [#permalink]

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04 Jul 2014, 08:59
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sagnik242 wrote:
whiplash2411 wrote:
udaymathapati wrote:
The sequence a1, a2, … , a n, … is such that an = 2an-1 - x for all positive integers n ≥ 2 and for certain number x. If a5 = 99 and a3 = 27, what is the value of x?
A. 3
B. 9
C. 18
D. 36
E.45

Quite simple to solve this one.

Given: $$a_n = 2a_{n-1} - x$$

$$a_5 = 99$$

$$a_3 = 27$$

$$a_5 = 2a_4 - x = 2(2a_3 - x) - x = 4a_3 - 3x = 99$$

$$4(27) - 3x = 99$$$$3x = 108-99 = 9$$

$$x = 3$$

QUESTION : How exactly did you get from 2(2a3 - x) to 4a3 * 3x, wouldn't it be 4a3 - 2x?

$$a_4=2a_3-x$$ --> $$a_5 = 2a_4 - x$$ --> $$a_4=2(2a_3-x)-x=4a_3-2x-x=4a_3-3x$$.
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Re: The sequence a1, a2, … , a n, … is such that an = 2an-1 - x [#permalink]

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23 Nov 2016, 09:20
Hello from the GMAT Club BumpBot!

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Re: The sequence a1, a2, … , a n, … is such that an = 2an-1 - x [#permalink]

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21 Jan 2017, 03:28
jet1445 wrote:
The sequence $$a_1$$, $$a_2$$, … , $$a_n$$, … is such that $$a_n = 2a_{n-1} - x$$ for all positive integers n ≥ 2 and for certain number x. If $$a_5 = 99$$ and $$a_3 = 27$$, what is the value of x?

A. 3
B. 9
C. 18
D. 36
E. 45

$$a_5= 2*a_4 - x = 99$$

$$a_4 = 2*a_3 - x = 2*27 - x$$

$$a_5 = 2*(54 - x ) -x = 99$$

108 - 3*x = 99

Therefore X = 3

Hence option A is correct.

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Re: The sequence a1, a2, … , a n, … is such that an = 2an-1 - x [#permalink]

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01 Aug 2017, 12:23
I did not understand the relation at first, but giving another shot worked well, thanks.
a5=99
a3=27

a5=2a4−x
=2(2a3−x)−x
=4a3−3x=99a5
=2a4−x
=2(2a3−x)−x
=4a3−3x=99

4(27)−3x
=994(27)−3x
=993x
=108−99
=93x
=108−99
=9

x=3
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Re: The sequence a1, a2, … , a n, … is such that an = 2an-1 - x   [#permalink] 01 Aug 2017, 12:23
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