Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2 : GMAT Problem Solving (PS)
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Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2

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Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2 [#permalink]

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21 Mar 2012, 04:18
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Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2^(n-1). What is the sum of the terms in sequence S when n=10?

A. 2^9
B. 2^10
C. 2^16
D. 2^35
E. 2^36

I think this is a weird question. First of all, shouldn't S1 be equal to 1 and not 2?

And even if S1 is 2, i still get 2^11 as the sum of all the terms.

source: gmathacks
[Reveal] Spoiler: OA

Last edited by Bunuel on 21 Mar 2012, 04:51, edited 1 time in total.
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Re: Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2 [#permalink]

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21 Mar 2012, 05:36
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BN1989 wrote:
Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2^(n-1). What is the sum of the terms in sequence S when n=10?

A. 2^9
B. 2^10
C. 2^16
D. 2^35
E. 2^36

I think this is a weird question. First of all, shouldn't S1 be equal to 1 and not 2?

And even if S1 is 2, i still get 2^11 as the sum of all the terms.

source: gmathacks

This question has quite a poor wording.

First of all: formula for $$n_{th}$$ term, $$S_n=2^{n-1}$$, should state that it's for $$n>1$$ (so for the second term and onward). Next I guess the question asks about the sum of the first 10 terms.

Given:
$$S_1=2$$;
$$S_2=2$$;
$$S_3=2^2$$;
$$S_4=2^3$$;
...
$$S_{10}=2^9$$

Question: $$2+2+2^2+2^3+...+2^9=?$$

Notice that: $$2+2=2^2$$ (the sum of the first 2 terms), $$2^2+2^2=2^3$$ (the sum of the first 3 terms), $$2^3+2^3=2^4$$ (the sum of the first 4 terms), so with similar logic the sum of the first 10 terms will be $$2^{10}$$.

Another approach:

We have the sum of 10 terms. Now, if all terms were equal to the largest term 2^9 we would have: $$sum=10*2^9\approx{2^4*2^9}=2^{13}$$, so the actual sum is less than $$2^{13}$$ but more than $$2^9$$ (option A). So the answer is clearly B.

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Re: Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2 [#permalink]

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10 Apr 2014, 03:06
Bunuel wrote:
BN1989 wrote:
Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2^(n-1). What is the sum of the terms in sequence S when n=10?

A. 2^9
B. 2^10
C. 2^16
D. 2^35
E. 2^36

I think this is a weird question. First of all, shouldn't S1 be equal to 1 and not 2?

And even if S1 is 2, i still get 2^11 as the sum of all the terms.

source: gmathacks

This question has quite a poor wording.

First of all: formula for $$n_{th}$$ term, $$S_n=2^{n-1}$$, should state that it's for $$n>1$$ (so for the second term and onward). Next I guess the question asks about the sum of the first 10 terms.

Given:
$$S_1=2$$;
$$S_2=2$$;
$$S_3=2^2$$;
$$S_4=2^3$$;
...
$$S_{10}=2^9$$

Question: $$2+2+2^2+2^3+...+2^9=?$$

Notice that: $$2+2=2^2$$ (the sum of the first 2 terms), $$2^2+2^2=2^3$$ (the sum of the first 3 terms), $$2^3+2^3=2^4$$ (the sum of the first 4 terms), so with similar logic the sum of the first 10 terms will be $$2^{10}$$.

Another approach:

We have the sum of 10 terms. Now, if all terms were equal to the largest term 2^9 we would have: $$sum=10*2^9\approx{2^4*2^9}=2^{13}$$, so the actual sum is less than $$2^{13}$$ but more than $$2^9$$ (option A). So the answer is clearly B.

From 2nd term this is becoming geometric sequence.

2+ 2(2^9-1)/2-1

2+( 2*2^9 -2)

2^10
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Re: Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2 [#permalink]

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18 Sep 2014, 11:14
BN1989 wrote:
Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2^(n-1). What is the sum of the terms in sequence S when n=10?

A. 2^9
B. 2^10
C. 2^16
D. 2^35
E. 2^36

I think this is a weird question. First of all, shouldn't S1 be equal to 1 and not 2?

And even if S1 is 2, i still get 2^11 as the sum of all the terms.

source: gmathacks

Sol:
I wanted to check what would be sum of 4 terms(n=4) based on that I can assume final answer:

2+2+2^2+2^3

2(1+1+2+2^2)= 2(2+2+2^2)= 2*2(1+1+2)= 2*2*2*2=2^4 =>2^n

so sum or n terms where n=10 willbe 2^10
Re: Sequence S is defined as follows: S1=2, S2=2^1, S3=2^2, SN=2   [#permalink] 18 Sep 2014, 11:14
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