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GMAT PREP question

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by stop@800 » Sat Dec 06, 2008 10:53 am
A small request: Please paste small attachments.
The image you pasted is 2.25 MB. You can reduce the quality of image to decrease the size.

Here goes my answer
The term 1 will be 1/2
term 2 will be -1/4

similarly 1/8 , -1/16 and so on

term1 + term2
= 1/2 -1/4 = 1/4

similarly next sum will be 1/8 - 1/16
= 1/8

and the series will go on

so your sum will lie between 1/4 and 1/2
Ans IMO D
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by poonam1279 » Sat Dec 06, 2008 11:19 am
Thanks for the explanation and also suggestion of pasting small images(I didn't check size of the image before posting. Thanks for mentioning). It is correct OA D.
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by stop@800 » Sat Dec 06, 2008 11:29 am
poonam1279 wrote:Thanks for the explanation and also suggestion of pasting small images(I didn't check size of the image before posting. Thanks for mentioning). It is correct OA D.
You are welcome :)

Actually I am on slow connection so it took lot of time downloading the image :)
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by logitech » Sat Dec 06, 2008 11:48 am
The series is:

(1/2 - 1/4) + (1/8 - 1/16) + (1/32 - 1/64) + (1/128 -1/256) + (1/512 - 1/1024)

As you can see from the pairs, the first pair is by far the most important pair.

The rest of the pairs are yielding smaller numbers.

(1/2 - 1/4) = 1/4

So the sum will be greater than 1/4. T>1/4

The sum of the second pair: 1/16
The sum of the third pair is : 1/64
The sum of the fourth pair is: 1/256
The sum of the fifth pair is :1/1024

As you can the even if the 3rd,4th and 5th sum was : 1/6

The rest of the pair would total to : 4 x 1/16 = 1/4

So the rest of the pairs will sum up < 1/4.

1/4 < TOTAL SUM < 1/4+1/4

1/4 < Total Sum < 1/2
LGTCH
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