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# PS;: SUM

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Senior Manager
Joined: 02 Feb 2004
Posts: 345
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15 Apr 2005, 07:52
00:00

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions

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PLZ EXPLAIN
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Director
Joined: 18 Feb 2005
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15 Apr 2005, 09:48
mirhaque wrote:
PLZ EXPLAIN

13 is the 7th term and 39 is the 20th term

Now we have sum of n terms is n^2

So take a difference of the squares of poisitions of 39 and 13 which is
400-49 = 351
VP
Joined: 25 Nov 2004
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16 Apr 2005, 17:06
gmat2me2 wrote:
13 is the 7th term and 39 is the 20th term
Now we have sum of n terms is n^2. So take a difference of the squares of poisitions of 39 and 13 which is 400-49 = 351

gmat2me2, you have right approach. but you overlooked and miscalculated. it says including 13 and 39. it should be =20^2-6^2=364.

or alternatively, sum = n(F+L)/2 = 14(13+39)/2 = 14x26=364
Director
Joined: 18 Feb 2005
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16 Apr 2005, 19:43
MA wrote:
gmat2me2 wrote:
13 is the 7th term and 39 is the 20th term
Now we have sum of n terms is n^2. So take a difference of the squares of poisitions of 39 and 13 which is 400-49 = 351

gmat2me2, you have right approach. but you overlooked and miscalculated. it says including 13 and 39. it should be =20^2-6^2=364.

or alternatively, sum = n(F+L)/2 = 14(13+39)/2 = 14x26=364

This is what I am afraid of...... I try to do very quickly and landup making silly mistakes.......I should improve my strike rate

Thanks though for the catch
Re: PS;: SUM   [#permalink] 16 Apr 2005, 19:43
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