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# OG12 Mistake?

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Senior Manager
Joined: 08 Nov 2010
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06 May 2011, 10:24
00:00

Difficulty:

35% (medium)

Question Stats:

81% (00:26) correct 19% (01:05) wrong based on 16 sessions

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Hey all

I am referring to a question that was posted here before but im opening a new thread bc i rly need some urgent help with this very simple question.

question 36 from the OG.

for all integers N, the function F is defined by f(n)=a^n
where a is a constant. what is the value of f(1)

1. f(2)=100
2. f(3)=-1000

so,
this is how i started to solve and this is the problem.
plz let me know if im wrong somewhere or OG rly have a mistake.

THE ONLY INTEGER defined is N not A.
this is the basic mistake. so:

F(2)=10^2=a^2
so a=10/-10 BUT
it can also mean that a=1/10 and n=-2

same with f(3)=-1000= -10^3 OR -1/10^-3

so the answer for this question should be E not B.

am i wrong?

thanks!
[Reveal] Spoiler: OA

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06 May 2011, 10:54
1
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Expert's post
144144 wrote:
for all integers N, the function F is defined by f(n)=a^n
where a is a constant. what is the value of f(1)

1. f(2)=100
2. f(3)=-1000

F(2)=10^2=a^2
so a=10/-10 BUT
it can also mean that a=1/10 and n=-2

More importantly, there is a mathematical mistake in what you are doing. If

f(n) = a^n

then when we evaluate f(2), we replace n with 2 on the right side of the equation above:

f(2) = a^2

If instead we replace n with -2 to get a^(-2), as you have done, then we are finding the value of something completely different - we're finding the value of f(-2). The value of f(-2) is not relevant in this question since we aren't given any information about it.

I've gone through the entirety of the Quant material in OG-12 looking for errors or typos. There are a few in the solutions section (some typos, but also some mathematical or logical errors), and there are a couple of minor errors in the explanatory material at the beginning of the book. As best I can tell, the questions themselves are free of mistakes in OG-12 (though the first printing of OG-11 had a couple of misprints in the questions section). On the whole it's a very well-edited book, at least relative to most GMAT publications.
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Senior Manager
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07 May 2011, 11:43
thanks. Just to make sure i understand.

When GMAT is talking about F(x), we are talking about only X=integer AND >0
is that correct?
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07 May 2011, 11:53
144144 wrote:
thanks. Just to make sure i understand.

When GMAT is talking about F(x), we are talking about only X=integer AND >0
is that correct?

no..it depends upon the question given.

f(x) = 10x

f(0.1) = 1

f(1) = 10

f( -1) = -10
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08 May 2011, 20:50
B gives a= -10 Hence.
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Re: OG12 Mistake?   [#permalink] 08 May 2011, 20:50
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