Check GMAT Club Decision Tracker for the Latest School Decision Releases https://gmatclub.com/AppTrack

 It is currently 29 May 2017, 23:57

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# S is an infinite sequence, in which the value of a

Author Message
TAGS:

### Hide Tags

Manager
Joined: 11 Dec 2010
Posts: 115
WE: Consulting (Consulting)
Followers: 4

Kudos [?]: 39 [1] , given: 50

S is an infinite sequence, in which the value of a [#permalink]

### Show Tags

23 Sep 2011, 23:23
1
KUDOS
3
This post was
BOOKMARKED
00:00

Difficulty:

95% (hard)

Question Stats:

24% (02:01) correct 76% (01:22) wrong based on 214 sessions

### HideShow timer Statistics

S is an infinite sequence, in which the value of a particular member of the sequence is equal to the difference between the two preceding members of the sequence. What is the value of the 43rd member of the sequence?

(1) The 1st term is 1.
(2) The 3rd term is 3.
[Reveal] Spoiler: OA
Current Student
Joined: 26 May 2005
Posts: 564
Followers: 18

Kudos [?]: 215 [2] , given: 13

Re: DS Question - Sequences [#permalink]

### Show Tags

24 Sep 2011, 00:08
2
KUDOS
1
This post was
BOOKMARKED
gmatprep2011 wrote:
S is an infinite sequence, in which the value of a particular member of the sequence is equal to the difference between the two preceding members of the sequence. What is the value of the 43rd member of the sequence?

(1) The 1st term is 1.
(2) The 3rd term is 3.

St1: t1 = 1
Let t2 = x
t3 = t2-t1= x-1 ( difference between the two preceding members of the sequence)
t4 = t3-t2 = x-1-x = -1
t5 = t4-t3 = -1-x+1= -x
t6 = t5 -t4 = -x+1 = 1-x
t7 = t6-t5 = 1-x+x = 1

If u carefully see , this follows are particular pattern; first 3 terms , then next 3 ( but negative).. this is symmetry .. and will continue ( t7 will start again with 1)
hence t 7 = 1
t 13 = 1
t19=1
t25= 1
t31= 1
t 37= 1
t 43 = 1

Hence statement 1 : sufficient

St 2: not much of help .

hence its A.
Manager
Joined: 11 Dec 2010
Posts: 115
WE: Consulting (Consulting)
Followers: 4

Kudos [?]: 39 [1] , given: 50

Re: DS Question - Sequences [#permalink]

### Show Tags

24 Sep 2011, 00:16
1
KUDOS
@sudhir18n

It does not mention whether second term minus first term or vice versa... Can it be C or E then?
Math Forum Moderator
Joined: 20 Dec 2010
Posts: 2013
Followers: 163

Kudos [?]: 1832 [1] , given: 376

Re: DS Question - Sequences [#permalink]

### Show Tags

24 Sep 2011, 06:15
1
KUDOS
gmatprep2011 wrote:
@sudhir18n

It does not mention whether second term minus first term or vice versa... Can it be C or E then?

That's a valid point. However, it should also mention n>=3, because we need at least two terms to even execute this condition.

With the above assumption, "C" would be the answer.

But, if the question were written like this:

S is an infinite sequence, in which each term $$S_n=S_{(n-1)}-S_{(n-2)}$$, such that n>=3. What is the value of the 43rd member of the sequence?

1. $$S_1=1$$
2. $$S_3=3$$

Answer would be A as nicely explained by sudhir18n.
_________________
Manager
Status: How easy it is?
Joined: 09 Nov 2012
Posts: 121
Location: India
Concentration: Operations, General Management
GMAT 1: 650 Q50 V27
GMAT 2: 710 Q49 V37
GPA: 3.5
WE: Operations (Other)
Followers: 2

Kudos [?]: 80 [0], given: 174

Re: DS Question - Sequences [#permalink]

### Show Tags

08 May 2013, 20:45
sudhir18n wrote:
gmatprep2011 wrote:
S is an infinite sequence, in which the value of a particular member of the sequence is equal to the difference between the two preceding members of the sequence. What is the value of the 43rd member of the sequence?

(1) The 1st term is 1.
(2) The 3rd term is 3.

St1: t1 = 1
Let t2 = x
t3 = t2-t1= x-1 ( difference between the two preceding members of the sequence)
t4 = t3-t2 = x-1-x = -1
t5 = t4-t3 = -1-x+1= -x
t6 = t5 -t4 = -x+1 = 1-x
t7 = t6-t5 = 1-x+x = 1

If u carefully see , this follows are particular pattern; first 3 terms , then next 3 ( but negative).. this is symmetry .. and will continue ( t7 will start again with 1)
hence t 7 = 1
t 13 = 1
t19=1
t25= 1
t31= 1
t 37= 1
t 43 = 1

Hence statement 1 : sufficient

St 2: not much of help .

hence its A.

I could get to the answer, but took almost 2:30 to evaluate both the options. Can anyone please provide a quicker solution?
Manager
Joined: 17 Oct 2012
Posts: 58
Followers: 0

Kudos [?]: 8 [0], given: 0

Re: DS Question - Sequences [#permalink]

### Show Tags

28 May 2013, 08:35
With the above assumption, "C" would be the answer.

But, if the question were written like this:

S is an infinite sequence, in which each term $$S_n=S_{(n-1)}-S_{(n-2)}$$, such that n>=3. What is the value of the 43rd member of the sequence?

1. $$S_1=1$$
2. $$S_3=3$$

Why would the answer be c? you could have a sequence: 1,-2,3 or you could have 1,4,3. So the fourth term would be different in these two cases!! What is the source of this question??
Answer would be A as nicely explained by sudhir18n.[/quote]
Math Expert
Joined: 02 Sep 2009
Posts: 39066
Followers: 7759

Kudos [?]: 106594 [2] , given: 11630

Re: S is an infinite sequence, in which the value of a [#permalink]

### Show Tags

28 May 2013, 10:13
2
KUDOS
Expert's post
2
This post was
BOOKMARKED
gmatprep2011 wrote:
S is an infinite sequence, in which the value of a particular member of the sequence is equal to the difference between the two preceding members of the sequence. What is the value of the 43rd member of the sequence?

(1) The 1st term is 1.
(2) The 3rd term is 3.

Similar questions to practice:
if-s-is-the-infinite-sequence-s1-9-s2-99-s3-999-sk-10-k-127798.html
an-infinite-sequence-of-positive-integers-is-called-a-127696.html
an-infi-nite-sequence-of-positive-integers-is-called-a-127496.html

Hope it helps.
_________________
Intern
Joined: 23 Apr 2013
Posts: 6
Followers: 0

Kudos [?]: 1 [0], given: 0

Re: S is an infinite sequence, in which the value of a [#permalink]

### Show Tags

03 Jul 2013, 08:04
Hello.. Can we say that as the question stands now, the answer is "E". I feel so as there is no way to know whether t3= t2-t1 or t3= t1-t2.... Thanks in advance!
Manager
Joined: 24 Oct 2013
Posts: 175
Schools: LBS '18
GMAT 1: 720 Q49 V38
WE: Design (Transportation)
Followers: 9

Kudos [?]: 39 [0], given: 83

S is an infinite sequence, in which the value of a [#permalink]

### Show Tags

21 Sep 2014, 09:56
Bunuel wrote:
gmatprep2011 wrote:
S is an infinite sequence, in which the value of a particular member of the sequence is equal to the difference between the two preceding members of the sequence. What is the value of the 43rd member of the sequence?

(1) The 1st term is 1.
(2) The 3rd term is 3.

Similar questions to practice:
if-s-is-the-infinite-sequence-s1-9-s2-99-s3-999-sk-10-k-127798.html
an-infinite-sequence-of-positive-integers-is-called-a-127696.html
an-infi-nite-sequence-of-positive-integers-is-called-a-127496.html

Hope it helps.

Bunuel,

Why do we assume here that the difference = a2 - a1? couldn't it be equal to a1-a2 also?

First term is 1 , 3rd term is 3.

So the second term can be 4 OR -2, isn't it?

when you say 'difference between two numbers' it can either be x-y or y-x, right?
Math Expert
Joined: 02 Sep 2009
Posts: 39066
Followers: 7759

Kudos [?]: 106594 [0], given: 11630

Re: S is an infinite sequence, in which the value of a [#permalink]

### Show Tags

21 Sep 2014, 11:41
gauravkaushik8591 wrote:
Bunuel wrote:
gmatprep2011 wrote:
S is an infinite sequence, in which the value of a particular member of the sequence is equal to the difference between the two preceding members of the sequence. What is the value of the 43rd member of the sequence?

(1) The 1st term is 1.
(2) The 3rd term is 3.

Similar questions to practice:
if-s-is-the-infinite-sequence-s1-9-s2-99-s3-999-sk-10-k-127798.html
an-infinite-sequence-of-positive-integers-is-called-a-127696.html
an-infi-nite-sequence-of-positive-integers-is-called-a-127496.html

Hope it helps.

Bunuel,

Why do we assume here that the difference = a2 - a1? couldn't it be equal to a1-a2 also?

First term is 1 , 3rd term is 3.

So the second term can be 4 OR -2, isn't it?

when you say 'difference between two numbers' it can either be x-y or y-x, right?

This question is marked as poor quality. So, you could ignore it. Check similar questions in my post above.
_________________
Re: S is an infinite sequence, in which the value of a   [#permalink] 21 Sep 2014, 11:41
Similar topics Replies Last post
Similar
Topics:
6 An infinite sequence of positive integers is called a "beta 5 27 Sep 2016, 00:38
1 An infinite sequence of positive integers is called a 5 23 Jul 2016, 10:07
32 An infinite sequence of positive integers is called a 12 21 Nov 2016, 00:31
15 An infinite sequence of positive integers is called a 9 09 Jun 2016, 16:12
5 An infinite sequence of positive integers is called an "alph 20 12 Sep 2013, 08:35
Display posts from previous: Sort by