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# I run into this problem and have no clue what to do and how

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I run into this problem and have no clue what to do and how [#permalink]  28 Sep 2008, 15:15
I run into this problem and have no clue what to do and how to approach it (if you answer please explain the steps)

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 ?

one
two
three
four
five
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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 16:43

i am not sure but i solved by taking two random series.

1:1 ,-2 (total 1 negative product 1*-2 & 1 sign change)

2: 1,3,-2 (total 1 negative product ,3*-2 & 1 sign change)

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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 17:14
sorry buddy thats not the answer
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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 17:23
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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 17:32

In the sequence 1, -3, 2, 5, -4, -6

This sign changes 3 times ( therefore product of 2 adjacent numbers is negative )

1 to -3
-3 to 2
5 to -4

What's OA ?
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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 17:36
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 ?

one
two
three
four
five

(1*-3)=-3 negative
(-3*2)=-6 negative
(2*5)=10 positive
(5*-4)=-20 negative
(-4*-6)=24 positive
3 Negative products of consecutive terms
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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 17:37
IMO three
OA ?

if correct could expand on my method
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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 18:10
i am going with 3..

the number of sign changes..lets see...1, -3, 2, 5, -4, -6

-1*3=-3 -4*2=-8 -6*2=-12
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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 18:16
OA is three

I also guessed this answer but cannot explain it. Seems like bertlacy comes w logical explanation
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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 20:47
oops.. big blunder by me.. i have not see the series and just start calculating
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Re: Sequence problem - Nightmare [#permalink]  28 Sep 2008, 20:58
bertlacy wrote:
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 ?

one
two
three
four
five

(1*-3)=-3 negative
(-3*2)=-6 negative
(2*5)=10 positive
(5*-4)=-20 negative
(-4*-6)=24 positive
3 Negative products of consecutive terms

Agree that 3 is correct.
Re: Sequence problem - Nightmare   [#permalink] 28 Sep 2008, 20:58
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