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junker
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3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 Updated on: 26 Sep 2013, 08:13
Originally posted by junker on 22 Dec 2009, 08:17.
Last edited by Bunuel on 26 Sep 2013, 08:13, edited 2 times in total.
Renamed the topic, edited the question and added the OA.
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Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!

Difficulty:

55% (hard)

Question Stats:

64% (02:16) correct 36% (02:04) wrong based on 69 sessions

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3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 + 2 × 3^7 =

A. 3^7
B. 3^8
C. 3^14
D. 3^28
E. 3^30

OPEN DISCUSSION OF THIS QUESTION IS HERE: 3-108690.html

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xcusemeplz2009
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3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 22 Dec 2009, 10:03
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SOMETHING WRNG WITH PROBLEM PLS CHECK
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3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 23 Dec 2009, 13:36
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Hi,
The answer choices seem wrong.
This simplifies to 3^8
You begin with combining common terms, here 3

For eg:
3+3+3+2*3^2+2*3^3+...)
3(1+1+1+2*3...)
3(3+2*3...)
3^2(1+2...)

Since this is a geometric progression, you can use the sum of a GP to sum this series
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3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 26 Sep 2013, 08:06
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junker
3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 + 2 × 3^7 =

• 3^7
• 3^8
• 3^14
• 3^28
• 3^30

Cant remember where I had found this question (was in my revision notes), whats an elegant way to solve this?
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Anyone found a good approach for this one?
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3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 26 Sep 2013, 08:14
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3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 + 2 × 3^7 =

A. 3^7
B. 3^8
C. 3^14
D. 3^28
E. 3^30

We have the sum of 9 terms. Now, if all terms were equal to the largest term 2*3^7 we would have: sum=9*(2*3^7)=2*3^9=~3^10, so the actual sum is less than 3^10 and more than 3^7 (option A) as the last term is already more than that. So the answer is clearly B.

Answer: B.

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OPEN DISCUSSION OF THIS QUESTION IS HERE: 3-108690.html

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This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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Bunuel
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