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Help Needed Prep Question

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Senior Manager
Joined: 12 Oct 2008
Posts: 488

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18 Jul 2009, 10:00
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Help Needed Prep Question

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Manager
Joined: 03 Jul 2009
Posts: 103
Location: Brazil
Re: Help Needed Prep Question [#permalink]

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18 Jul 2009, 13:01
I got this questions while doing the simulation as well.

This is what I did.

The first part of the equation, $$(-1)^k+1$$, will only determine whether the number is positive or negative.

For k = 1, this part will be 1
For k = 2, this part will be -1
For k = 3, this part will be 1, again. Thus, when k is even the term will be multiple by -1, and it will be negative. However, when k is odd the term will be positive.

The second part of the equation just tell us that is a fraction of power of 2.

Thus the numbers are:

$$1/2, -1/4, 1/8, -1/16, 1/32, -1/64, 1/128, -1/256, 1/512, -1/1024$$

Now try to sum the first positives to have a impression of the sum of the positives, and do the same with the negatives.

If you transform them to decimal, you will notice that the first positives, are, 0.5, 0.125, 0.03, now hold on. The sum here is 0.655. The way the numbers are going down to fast, you can see that this sum will not pass 0.7, or if it pass, it will be very close to 0.7. Save this number.

Now, if you sum the negatives, in decimal, you will see -0.25, -0.06, ok that is enough. You do not need even to continue. As you can see, again the numbers are going down ("modularity talking") so fast that you can see the the sum will be around -0.3.

Thus $$0.7 - 0.3 =~ 0.4$$, your answer approximately.

D between 0.25 and 0.5

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: Help Needed Prep Question   [#permalink] 18 Jul 2009, 13:01
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