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Manager
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I found it quite interesting and they said this is one of [#permalink]
 05 Jun 2006, 02:07
I found it quite interesting and they said this is one of the most difficult questions in Gmat. Be careful 
Sequence S consists of 24 non-zero intergers. From the third one, every term of S is the product of the previous 2 terms. How many terms in S are negative?
(1) The third term in S is Positive
(2) The fourth term in S is negative
Good luck!!!
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Intern
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B it is.
(1) The third term in S is Positive
it gives two possible sequesnces: -
(+ve, +ve ,+ve .............+ve) or (-ve, -ve, +ve, -ve, -ve, +ve........)
so we can't count the number of -ve integers.
(2) The fourth term in S is negative
It gives the following sequences: -
(-ve, +ve, -ve, -ve, +ve, -ve, -ve, +ve, -ve, ......so on)
or
(-ve, -ve, +ve, -ve, -ve, +ve, -ve, -ve, +ve, ......so on)
If you see, in both the above series a pattern is repeated after 3 numbers, i.e. 2 negative and 1 positive numbers. So there are 16 -ve numbers in the series.
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Can someone pls explain how they got B? I did not get the explanations above.
Here is the approach I took:
let the seq be:
n1, n2, n3, n4, n5......
n3 = n1*n2
n4= n1*n2*n2
From St. 2
The fourth term is -ve,
Which means n1 is -ve and n2^2 is +ve. However we still do not know what sign n2 is.
If you take both statements together it works.
Am I missing something here?
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Intern
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Quote: n4= n1*n2*n2
It says every term is the product of previous two terms
so, n4 = n3 * n2, n5 = n4 * n3 and so on.
Now, Please read the above explanation and let me know.
_________________
If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut.
Albert Einstein
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Manager
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mendiratta_1812 wrote: Quote: n4= n1*n2*n2 It says every term is the product of previous two terms so, n4 = n3 * n2, n5 = n4 * n3 and so on. Now, Please read the above explanation and let me know. I am so sorry unfortunately I still don't get it. With my test 2 weeks away this is not good.
I agree n4 = n3*n2
but n3 = n1*n2
so n4 = n1*n2*n2---n2*n2 will be +ve
I guess what I don't get is how do we ascertain whether n2 is +ve or -ve?
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Director
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With A:
sequence can be {-,-,+,-...} or {+,+,+,...}
With B:
sequence can be {-,+,-,-,+,-,-,+,...} or {-,-,+,-,-,+...}
from here, we can calculate # of -ves
Hence B
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Manager
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agree with B
mendiratta_1812,
thanks for the warning
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Director
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But what about the first term, can it be determined if it is + or -. The stem says that each term after the third is the product
thanks
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"(2) The fourth term in S is negative
It gives the following sequences: -
(-ve, +ve, -ve, -ve, +ve, -ve, -ve, +ve, -ve, ......so on)
or
(-ve, -ve, +ve, -ve, -ve, +ve, -ve, -ve, +ve, ......so on) "
I think this is an excellent answer. Because of the repetition of every 3 terms with 2negative and 1 positive so we can get the answer from (2). And the OA is obviously B.
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close
