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Re: Is f(n) > f(n 1)? (1) n = 8 (2) f(n) = n - 1 [#permalink]
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1. No information about the function itself. f(n) could be more or less than f(n-1). NOT SUFFICIENT
2. Gives us the general function. From this we have sufficient information to test the inequality. SUFFICIENT

ANSWER D



zxcvbnmas wrote:
Is f(n)>f(n−1)?

(1) n=8

(2) f(n) = n-1
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Re: Is f(n) > f(n 1)? (1) n = 8 (2) f(n) = n - 1 [#permalink]
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zxcvbnmas wrote:
Is f(n) > f(n−1)?

(1) n = 8

(2) f(n) = n-1


Target question: Is f(n) > f(n−1)?

Statement 1: n = 8
Plug n = 8 into target question to get: Is f(8) > f(8−1)?
In other words, Is f(8) > f(7)?
We cannot answer the target question with certainty, because we don't know anything about the function f.
So, statement 1 is NOT SUFFICIENT

Statement 1: f(n) = n-1
At first glance, we might assume that statement 2 is not sufficient, since we don't know the value of n.
However, now that we know how the function f behaves, we can, indeed answer the target question.
If f(n) = n-1, then...
f(n) = n - 1 and f(n - 1) = (n - 1) - 1 = n - 2
So, the original target question Is f(n) > f(n−1)? becomes Is n - 1 > n − 2?
Sure, n - 1 is definitely greater than n - 2
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Answer = B

Originally posted by BrentGMATPrepNow on 26 Aug 2016, 12:05.
Last edited by BrentGMATPrepNow on 07 Jul 2020, 18:25, edited 1 time in total.
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Re: Is f(n) > f(n 1)? (1) n = 8 (2) f(n) = n - 1 [#permalink]
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zxcvbnmas wrote:
Is f(n)>f(n−1)?

(1) n=8

(2) f(n) = n-1


Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

Since we have 1 variable (n) and 0 equations, D is most likely to be the answer. So, we should consider each of the conditions on their own first.

Condition 1) : n = 8
Since f(n) is not determined, condition 1) is not sufficient.

Condition 2) : f(n) = n - 1
Since f(n) = n - 1, we have f(n-1) = (n-1)-1 = n-2.
Since n - 1 > n - 2, f(n) > f(n-1).
Thus, the condition 2) is sufficient.

Therefore, B is the answer.

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A,B,C or E.
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Re: Is f(n) > f(n 1)? (1) n = 8 (2) f(n) = n - 1 [#permalink]
zxcvbnmas wrote:
Is f(n)>f(n−1)?

(1) n=8

(2) f(n) = n-1


(1) We can rephrase the question as \(f(8)>f(7)\)?

However, we have no information regarding the function. INSUFFICIENT.

(2) We have \(n - 1 > (n - 1) - 1\)
\(n - 1 > n -2\)

SUFFICIENT.
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Re: Is f(n) > f(n 1)? (1) n = 8 (2) f(n) = n - 1 [#permalink]
BrentGMATPrepNow wrote:
zxcvbnmas wrote:
Is f(n) > f(n−1)?

(1) n = 8

(2) f(n) = n-1


Target question: Is f(n) > f(n−1)?

Statement 1: n = 8
Plug n = 8 into target question to get: Is f(8) > f(8−1)?
In other words, Is f(8) > f(7)?
We cannot answer the target question with certainty, because we don't know anything about the function f.
So, statement 1 is NOT SUFFICIENT

Statement 1: f(n) = n-1
At first glance, we might assume that statement 2 is not sufficient, since we don't know the value of n.
However, now that we know how the function f behaves, we can, indeed answer the target question.
If f(n) = n-1, then...
f(n) = n - 1 and f(n - 1) = (n - 1) - 1 = n - 2
So, the original target question Is f(n) > f(n−1)? becomes Is n - 1 > n − 2?
Sure, n - 1 is definitely greater than n - 2
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Answer = B



BrentGMATPrepNow in ST 1 , f(8) > f(7)? what do you mean "because we don't know anything about the function f." we know what "n" is.... Can you give example how in this case function may behave :)

i thought St 1 is sufficient ... i just wrote 8>7 :lol:

Any videos on functions ? :)
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Re: Is f(n) > f(n 1)? (1) n = 8 (2) f(n) = n - 1 [#permalink]
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dave13 wrote:
BrentGMATPrepNow wrote:
zxcvbnmas wrote:
Is f(n) > f(n−1)?

(1) n = 8

(2) f(n) = n-1


Target question: Is f(n) > f(n−1)?

Statement 1: n = 8
Plug n = 8 into target question to get: Is f(8) > f(8−1)?
In other words, Is f(8) > f(7)?
We cannot answer the target question with certainty, because we don't know anything about the function f.
So, statement 1 is NOT SUFFICIENT

Statement 1: f(n) = n-1
At first glance, we might assume that statement 2 is not sufficient, since we don't know the value of n.
However, now that we know how the function f behaves, we can, indeed answer the target question.
If f(n) = n-1, then...
f(n) = n - 1 and f(n - 1) = (n - 1) - 1 = n - 2
So, the original target question Is f(n) > f(n−1)? becomes Is n - 1 > n − 2?
Sure, n - 1 is definitely greater than n - 2
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Answer = B



BrentGMATPrepNow in ST 1 , f(8) > f(7)? what do you mean "because we don't know anything about the function f." we know what "n" is.... Can you give example how in this case function may behave :)

i thought St 1 is sufficient ... i just wrote 8>7 :lol:

Any videos on functions ? :)


Here's an example:

f(n) = 2 - n
(1) n = 8
So, f(n) = f(8) = 2 - 8 = -6
And f(n-1) = f(7) = 2 - 7 = -5
In this case, f(n-1) > f(n)

Now consider the function f(n) = 2n
(1) n = 8
So, f(n) = f(8) = (2)(8) = 16
And f(n-1) = f(7) = (2)(7) = 14
In this case, f(n) > f(n-1)

Does that help?
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Re: Is f(n) > f(n 1)? (1) n = 8 (2) f(n) = n - 1 [#permalink]
BrentGMATPrepNow wrote:
zxcvbnmas wrote:
Is f(n) > f(n−1)?

(1) n = 8

(2) f(n) = n-1


Target question: Is f(n) > f(n−1)?

Statement 1: n = 8
Plug n = 8 into target question to get: Is f(8) > f(8−1)?
In other words, Is f(8) > f(7)?
We cannot answer the target question with certainty, because we don't know anything about the function f.
So, statement 1 is NOT SUFFICIENT

Statement 1: f(n) = n-1
At first glance, we might assume that statement 2 is not sufficient, since we don't know the value of n.
However, now that we know how the function f behaves, we can, indeed answer the target question.
If f(n) = n-1, then...
f(n) = n - 1 and f(n - 1) = (n - 1) - 1 = n - 2
So, the original target question Is f(n) > f(n−1)? becomes Is n - 1 > n − 2?
Sure, n - 1 is definitely greater than n - 2
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Answer = B


Hi BrentGMATPrepNow, For St.1 not sufficient, is it because it's being given as n = 8 and not f(n) = 8? Therefore function f is not being defined.
Could we understand it this way? Thanks Brent
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Re: Is f(n) > f(n 1)? (1) n = 8 (2) f(n) = n - 1 [#permalink]
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Kimberly77 wrote:

Hi BrentGMATPrepNow, For St.1 not sufficient, is it because it's being given as n = 8 and not f(n) = 8? Therefore function f is not being defined.
Could we understand it this way? Thanks Brent

That's correct.
If we don't have any information about what the function does, there's no way to answer the target question.
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Re: Is f(n) > f(n 1)? (1) n = 8 (2) f(n) = n - 1 [#permalink]
BrentGMATPrepNow wrote:
Kimberly77 wrote:

Hi BrentGMATPrepNow, For St.1 not sufficient, is it because it's being given as n = 8 and not f(n) = 8? Therefore function f is not being defined.
Could we understand it this way? Thanks Brent

That's correct.
If we don't have any information about what the function does, there's no way to answer the target question.


Brilliant thanks BrentGMATPrepNow for great explanation always. :thumbsup: :please:
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