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555-605 Level|   Sequences|      
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NoHalfMeasures
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If Sn is the sum of the first n terms of a certain sequence 03 Jun 2014, 18:50
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C
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74% (01:32) correct 26% (01:46) wrong based on 1210 sessions

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If Sn is the sum of the first n terms of a certain sequence and if Sn = n(n+1) for all positive integers n, what is the third term of the sequence?

A. 3
B. 4
C. 6
D. 8
E. 9
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C
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KarishmaB
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If Sn is the sum of the first n terms of a certain sequence 02 Dec 2014, 21:26
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angelfire213
I guess my question is how is S2 different from the 2nd term conceptually. I know to those who get it, this might seem obvious but I don't understand how S2 is 6 and the 2nd term is 4.

ie isn't S2 = the 2nd term?



You are getting confused with the variables.

Usually,

Tn = nth term
T1 = 1st term
T2 = 2nd term
and so on...

Sn = Sum of first n terms of the sequence
Sn = 1st term + 2nd term + 3rd term + ... + nth term
Sn = T1 + T2 + T3 + ....Tn

You are given here that "Sn is the SUM of first n terms..." So you have
Sn = T1 + T2 + T3 + ....Tn = n(n+1)

So S1 = T1
S2 = T1 + T2
S3 = T1 + T2 + T3
and so on

S1 = T1 = 1*(1+1) = 2
S2 = T1 + T2 = 2 + T2 = 2*(2+1) = 6
So T2 = 4

S3 = T1 + T2 + T3 = 2 + 4 + T3 = 3*(3+1) = 12
So T3 = 6
(This is what we wanted)

The third term is 6.
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If Sn is the sum of the first n terms of a certain sequence 03 Jun 2014, 18:54
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S3 = 3*4 = 12
S2 = 2*3 = 6

So the third element of seq = 12 - 6 = 6
Ans : C
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If Sn is the sum of the first n terms of a certain sequence 03 Jun 2014, 20:35
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Let there be 3 terms in the sequence, namely a, b & c

Given that a + b + c = 3 * 4 = 12 .............. (1)

Now say there are 2 terms, namely a & b

So, a + b = 2 * 3 = 6 .......... (2)

Substitute value of a+b from eqn (2) in eqn (1)

6 + c = 12

c = 6

Answer = C
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If Sn is the sum of the first n terms of a certain sequence 17 Nov 2014, 04:50
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Hi All,

I'm totally lost and unable to understand this question. :oops: :roll:

a) PareshGmat: where is this given in the question: Given that a + b + c = 3 * 4 = 12 .............. (1)
b) MensaNumber: how did you get to this:
S3 = 3*4 = 12
S2 = 2*3 = 6
So the third element of seq = 12 - 6 = 6
c) What does the third term mean?
d) How have we assumed the first number in the sequence to be 1? Is it because that equation holds true for all positive numbers? But the series could start with say a 101!

Any help would be greatly appreciated! :-D
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If Sn is the sum of the first n terms of a certain sequence 17 Nov 2014, 06:33
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Approach: Back Solve

Steps:
1. start with C (6)
2. Is 6 product of subsequent numbers ? - yes 6=2 x3
3 Check other options before confirmation - none of them are products of subsequent number

Answer:C
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If Sn is the sum of the first n terms of a certain sequence 17 Nov 2014, 11:55
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joseph0alexander
If Sn is the sum of the first n terms of a certain sequence and if Sn = n(n+1) for all positive integers n, what is the third term of the sequence?

A. 3
B. 4
C. 6
D. 8
E. 9

Hi All,

I'm totally lost and unable to understand this question. :oops: :roll:

a) PareshGmat: where is this given in the question: Given that a + b + c = 3 * 4 = 12 .............. (1)
b) MensaNumber: how did you get to this:
S3 = 3*4 = 12
S2 = 2*3 = 6
So the third element of seq = 12 - 6 = 6
c) What does the third term mean?
d) How have we assumed the first number in the sequence to be 1? Is it because that equation holds true for all positive numbers? But the series could start with say a 101!

Any help would be greatly appreciated! :-D

We are told that the sum of a certain sequence is given by the formula Sn = n(n+1). So, the sum of the first 2 terms is S2 = 2(2+1) = 6, the sum of the first 3 terms is S3 = 3(3 + 1) = 12 and so on. Basically to get the sum of the first n terms we simply should substitute the value of n in the formula.

Does this make sense?
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If Sn is the sum of the first n terms of a certain sequence 02 Dec 2014, 11:05
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i still don't get it. Are we talking 2 different "S"s here. It seams we are saying SN is the sum oft the first n terms and SN is also equal to n(n+1).

If we say S3 is 6 as the answer says, then n is 3 correct? SO S3 should be 3(3+1) = 12.

That's why i was confused on the question as to how to differentiate between the S's and N's.

any help please
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If Sn is the sum of the first n terms of a certain sequence 02 Dec 2014, 11:18
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angelfire213
i still don't get it. Are we talking 2 different "S"s here. It seams we are saying SN is the sum oft the first n terms and SN is also equal to n(n+1).

Nope. We are telling
a) SN is the sum oft the first n terms and
b) SN has the equation n(n+1)

Hope this helps! :)
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If Sn is the sum of the first n terms of a certain sequence 02 Dec 2014, 11:29
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Slightly different approach.
Sn=n(n+1)
now, suppose if the sequence has 1 term only. then sum will be equal to that term. so S1=first term
put n=1 in the expression we have
S1=1(1+1) =2
thus first term of the sequence =2
now S2=2(2+1)=6
we know that first term of the sequence =2, thus second term will be 6-2 =4
now, S3=3(3+1) =12,
thus third term will be 12-first term - second term =12-2-4 =6
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If Sn is the sum of the first n terms of a certain sequence 02 Dec 2014, 16:06
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I guess my question is how is S2 different from the 2nd term conceptually. I know to those who get it, this might seem obvious but I don't understand how S2 is 6 and the 2nd term is 4.

ie isn't S2 = the 2nd term?

(I know once the light bulb clicks i'll be like "ohhhhhhh" but help me understand please)

manpreetsingh86
Slightly different approach.
Sn=n(n+1)
now, suppose if the sequence has 1 term only. then sum will be equal to that term. so S1=first term
put n=1 in the expression we have
S1=1(1+1) =2
thus first term of the sequence =2
now S2=2(2+1)=6
we know that first term of the sequence =2, thus second term will be 6-2 =4
now, S3=3(3+1) =12,
thus third term will be 12-first term - second term =12-2-4 =6
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If Sn is the sum of the first n terms of a certain sequence 16 Sep 2016, 07:48
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NoHalfMeasures
If Sn is the sum of the first n terms of a certain sequence and if Sn = n(n+1) for all positive integers n, what is the third term of the sequence?

A. 3
B. 4
C. 6
D. 8
E. 9

let A1, A2, A3 be the first 3 terms
Sum of first 2 terms = A1+A2 = 2(2+1) = 6 ..............(i)
Sum of the three terms = A1+A2+A3 = 3(3+1) = 12 ......(ii)

(ii)-(i) = A3= 6
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If Sn is the sum of the first n terms of a certain sequence 05 Nov 2017, 08:34
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NoHalfMeasures
If Sn is the sum of the first n terms of a certain sequence and if Sn = n(n+1) for all positive integers n, what is the third term of the sequence?

A. 3
B. 4
C. 6
D. 8
E. 9

We see that S3 = 3(4) = 12 and S2 = 2(3) = 6. Since S3 is the sum of the first three terms and S2 is the sum of the first two terms, the difference, S3 - S2 = 12 - 6 = 6, must be the value of the third term of the sequence.

Answer: C
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If Sn is the sum of the first n terms of a certain sequence 25 Apr 2019, 20:32
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Hi,
There is a 15 Sec approach for this question

Well this is worth remembering that

Sum of first 'n' odd natural numbers =\(n^{2}\)
sum of first 'n' even natural numbers= n(n+1)or = \(n^{2}\)+n

So question is asking us for what is the third 3 even natural numbers ( 2,4, 6)

6 is the third even natural number .

Hope this helps
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If Sn is the sum of the first n terms of a certain sequence 14 Jul 2020, 23:43
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sum of first 'n' even natural numbers= n(n+1)

So question is asking us for what is the third 3 even natural numbers ( 2,4, 6)

chetan2u
My question is
Zero is an even integer, so shouldn't the first three terms be 0,2,4 ?
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If Sn is the sum of the first n terms of a certain sequence 15 Jul 2020, 00:36
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Evo23
sum of first 'n' even natural numbers= n(n+1)

So question is asking us for what is the third 3 even natural numbers ( 2,4, 6)

chetan2u
My question is
Zero is an even integer, so shouldn't the first three terms be 0,2,4 ?

Natural numbers start from 1, so 0 will not be counted.
Whole number start from 0.

However I doubt GMAT will deal with natural numbers, it will call it positive integers
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If Sn is the sum of the first n terms of a certain sequence 27 Dec 2023, 04:21
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Could anyone please clarify if S(n) - S(n-1) is always equal to a(n)?
Sum(nth term) - Sum(nth term -1) = nth term

Example:
S4-S3=a3
Similarly, S2-S1=a2
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If Sn is the sum of the first n terms of a certain sequence 27 Dec 2023, 06:19
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Deepam10
If Sn is the sum of the first n terms of a certain sequence and if Sn = n(n+1) for all positive integers n, what is the third term of the sequence?

A. 3
B. 4
C. 6
D. 8
E. 9

Could anyone please clarify if S(n) - S(n-1) is always equal to a(n)?
Sum(nth term) - Sum(nth term -1) = nth term

Example:
S4-S3=a3
Similarly, S2-S1=a2
Bunuel

Yes, S(n) - S(n-1) = a(n). This is because S(n) = S(n - 1) + a(n), so the difference S(n) - S(n-1) is obviously a(n). Here, S(n-1) is the sum of the first n-1 terms, so, when you add a(n) to tat you get the sum of n terms, which is S(n).
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If Sn is the sum of the first n terms of a certain sequence 04 Jan 2025, 20:23
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