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Sum of series

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Sum of series

by harsh.champ » Tue Feb 09, 2010 3:49 am
Find the sum of the series S, where

(A)13/3
(B)11/ 3
(C)25/ 6
(D)23 /6
(E)4

THe OA is D.
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Last edited by harsh.champ on Tue Feb 09, 2010 3:52 am, edited 1 time in total.
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by thephoenix » Tue Feb 09, 2010 3:52 am
harsh.champ wrote:Find the sum of the series S, where

(A)13/3
(B)11/ 3
(C)25/ 6
(D)23 /6
(E)4

THe OA is D.
i guess something is missing in the question
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by harsh.champ » Tue Feb 09, 2010 3:54 am
thephoenix wrote:
harsh.champ wrote:Find the sum of the series S, where

(A)13/3
(B)11/ 3
(C)25/ 6
(D)23 /6
(E)4

THe OA is D.
i guess something is missing in the question
Yes,I had attached a bitmap image which was not shown.
Now, I have changed it to jpeg image.
Hope you can find the series now.:)
It takes time and effort to explain, so if my comment helped you please press Thanks button :)



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by Ian Stewart » Tue Feb 09, 2010 1:08 pm
You can never see a question like this on the GMAT, for two reasons. First, the GMAT never tests whether you understand how to sum an infinite series. Second, they can never give you the first few terms of a sequence and ask you to guess what the next terms will be; they need to tell you some rule that allows you to logically work out what the later terms are. Otherwise it's just a matter of guessing what the question designer was thinking, and the GMAT does not test whether you are psychic. If you know the first few terms of a sequence, there are always many perfectly logical ways to continue the sequence. For example, if I provide the sequence:

4, 4, 2, 2, ...

what is the next term? It would be logical to think the next term was 0, of course, but it would also be logical to think the next term was 3 (the sequence might be giving the number of primes between 0 and 10, between 10 and 20, between 20 and 30, between 30 and 40, and so on). There are certainly other possibilities (perhaps a_n = 0.5*(a_(n-2)), and the next term is 1, for example). There is no correct answer. The same is true given the first few terms of any sequence, though it's more difficult to provide simple examples when the numbers are complicated, as in the question above.

In any case, the question above could never appear on the GMAT.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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