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For a finite sequence of nonzero numbers, the number of

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Director
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For a finite sequence of nonzero numbers, the number of [#permalink]

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23 May 2008, 14:02
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For a finite sequence of nonzero numbers, the number of variations in sign is defined as the number of pairs of consecutive terms of the sequence for which the product of the two consecutive terms is negative. What is the number of variations in sign for the sequence 1, -3, 2, 5, -4, -6?

a. one
b. two
c. three
d. four
e. five

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23 May 2008, 16:18
Variation in sign = consecutive numbers whose product is negative.

(1,-3), (-3,2), and (5,-4) satisfy the definition.

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Director
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23 May 2008, 18:47
YihWei, that is the correct answer...I got this simple one wrong because I ordered the set...why would one not order the set in this case? How would you reword this question if they DID want you to order the set?

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23 May 2008, 21:05
jimmyjamesdonkey wrote:
YihWei, that is the correct answer...I got this simple one wrong because I ordered the set...why would one not order the set in this case? How would you reword this question if they DID want you to order the set?

I didn't order the set because the question didn't ask me to do that. I just try to "play dumb" and do exactly what the question asks me to do and nothing more. If they did want us to order the set I would probably just add a statement to the end of the question saying, "Put the sequence in ascending/descending order prior to solving the question". I think this is just a simple case of you overthinking the question. Stop being smarter than the GMAT
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23 May 2008, 23:07
Jimmy,

Because this is NOT a set... it clearly states SEQUENCE. There are either Finite or Infinite sequences. In this case it was Finite so there is a set number of values.

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04 Jun 2008, 10:58
jimmyjamesdonkey wrote:
YihWei, that is the correct answer...I got this simple one wrong because I ordered the set...why would one not order the set in this case? How would you reword this question if they DID want you to order the set?

Ive been stumped on this question for soo long (I just memorized the answer and somehow convinced myself that the answer is 3 and not 1) - I did the same thing you did.

thanks for the clarification all.

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06 Jun 2008, 02:54
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Answer: C, couldn't put in better words than YihWei

YihWei wrote:
Variation in sign = consecutive numbers whose product is negative.

(1,-3), (-3,2), and (5,-4) satisfy the definition.

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06 Jun 2008, 15:26
Or just think about what they are asking for in common sense terms. The "number of variations in sign" is how many times the sequence flips between positive and negative numbers. So it starts positive at 1, then flips once at -3, then flips a second time at 2, then stays the same at 5, then flips a third time at -4, then stays the same at -6. You could get the answer without even looking at the numerals, just the signs. +-++-- = 3 flips.

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07 Jun 2008, 10:01
tritium6 wrote:
Or just think about what they are asking for in common sense terms. The "number of variations in sign" is how many times the sequence flips between positive and negative numbers. So it starts positive at 1, then flips once at -3, then flips a second time at 2, then stays the same at 5, then flips a third time at -4, then stays the same at -6. You could get the answer without even looking at the numerals, just the signs. +-++-- = 3 flips.

nice way of doing this..but i gurantee on exam day under the stress from the exam..one will most likely get this wrong cause they will most likely over look one of the - signs or + signs..and under pressure to hurry will make a careless mistake..

best approach is laid out by YihWei..

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08 Jun 2008, 16:33
jimmyjamesdonkey wrote:
YihWei, that is the correct answer...I got this simple one wrong because I ordered the set...why would one not order the set in this case? How would you reword this question if they DID want you to order the set?

Jimmy I did the same things as well, I ordered the sequence which screwed me!

A few takeaways from this problem:

1). Dont assume anything, in this case the assumption made was to order the sequence in ascending order when not explicitly told to do so
2). Pay attention to Detail - As chengliu pointed out, this is NOT a set, rather a sequence, so I can see how ordering the set might make sense in that case, but once again something should state/trigger that action. (For this problem, "consecutive" was the keyword that screwed me. In a set consecutive numbers means they are either ordered in ascending/descending order. However in a sequence, depending on the sequence pattern - consecutive numbers are NOT necessarily ordered in ascending/descending order.

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Re: GMATPrep   [#permalink] 08 Jun 2008, 16:33
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For a finite sequence of nonzero numbers, the number of

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