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In the sequence S of numbers, each term after the first two

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In the sequence S of numbers, each term after the first two [#permalink] New post 30 May 2011, 04:54
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In the sequence S of numbers, each term after the first two terms is the sum of the two immediately
preceding terms. What is the 5th term of S?

(1) The 6th term of Sminus the 4th termequals 5.
(2) The 6th term of Splus the 7th term equals 21.


OPEN DISCUSSION OF THIS QUESTION IS HERE: in-the-sequence-s-of-numbers-each-term-after-the-first-two-167947.html
[Reveal] Spoiler: OA
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Re: In the sequence S of numbers, each term after the first two [#permalink] New post 30 May 2011, 09:07
Let first term be x and 2nd be y.
So,

1st term 2nd term 3rd term 4th term 5th term 6th term 7th term
x y x+y x+2y 2x+3y 3x+5y 5x+8y

st-1 ==> 3x+5y-x-2y = 5
so 2x+3y = 5
which is your 5th term
Sufficient

st-2 ==> 3x+5y+5x+8y = 8x+13y=21
you cannot deduce it to 2x+3y.
not sufficient

Answer A.
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Re: In the sequence S of numbers, each term after the first two [#permalink] New post 02 Jun 2011, 16:52
st-2 ==> 3x+5y+5x+8y = 8x+13y=21
you cannot deduce it to 2x+3y.
not sufficient


can you please explain the highlighted part ??
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Re: In the sequence S of numbers, each term after the first two [#permalink] New post 02 Jun 2011, 18:44
shashankp27 wrote:
st-2 ==> 3x+5y+5x+8y = 8x+13y=21
you cannot deduce it to 2x+3y.
not sufficient


can you please explain the highlighted part ??


2nd statement reads - (2) The 6th term of Splus the 7th term equals 21.
And its asking the value of 5th term..which in the sequence i laid out is 2x+3y

now, by the above explanation, we deduced the sum of 6th term and 7th term as 8x+13y = 21.
from this, we cannot derive the value of 2x+3y.. so its not sufficient.

hope it helps.

or example - may think that, if suppose sum of 6th term and 7th term is 4x+6y = 21
so from this you can find the value of 5th term as 2x+3y = 21/2 = 10.5
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Re: In the sequence S of numbers, each term after the first two [#permalink] New post 14 Jul 2015, 05:48
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Re: In the sequence S of numbers, each term after the first two [#permalink] New post 14 Jul 2015, 07:46
Expert's post
conan9118 wrote:
In the sequence S of numbers, each term after the first two terms is the sum of the two immediately
preceding terms. What is the 5th term of S?

(1) The 6th term of Sminus the 4th termequals 5.
(2) The 6th term of Splus the 7th term equals 21.


In the sequence S of numbers, each term after the first two terms is the sum of the two immediately preceding terms. What is the 5th term of S?

\(s_n=s_{n-1}+s_{n-2}\), for \(n>2\).

(1) The 6th term of S minus the 4th term equals 5 --> \(s_6-s_4=5\) --> \((s_5+s_4)-s_4=5\) --> \(s_5=5\). Sufficient.

(2) The 6th term of S plus the 7th term equals 21 --> \(s_6+s_7=21\) --> \(s_6+(s_6+s_5)=21\). Since we don't know \(s_6\) we cannot find \(s_5\). Not sufficient.

Answer: A.

OPEN DISCUSSION OF THIS QUESTION IS HERE: in-the-sequence-s-of-numbers-each-term-after-the-first-two-167947.html
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Re: In the sequence S of numbers, each term after the first two   [#permalink] 14 Jul 2015, 07:46
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