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EIVs - Formulas Question

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tsuttonc
 Post subject: EIVs - Formulas Question
 Post Posted: Mon Jul 20, 2009 10:33 pm 
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Course Students


Posts: 1
GMATPrep Software Practice Test Question...

For which of the following functions is f(a+b) = f(a)+f(b) for all positive numbers a and b?

A) f(x) = x^2
B) f(x) = x+1
C) f(x) = √x
D) f(x) = 2/x
E) f(x) = -3x

The answer is E, but I'm not sure why and the GMATPrep Software doesn't give an explanation. Could someone please help me me understand? Thanks!
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mxs2009
 Post subject: Re: EIVs - Formulas Question
 Post Posted: Tue Jul 21, 2009 12:29 am 
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Students


Posts: 16
Because this is a problem solving question, and the stem states, "for all positive numbers a and b," I would just pick numbers. a=1 b=3

A. f(1+3) = f(1)+f(3)
4^2 = 1^2 + 3^2
16=2+9 => NO
B. (4+1) = (1+1) + (3+1)
5=2+4 => NO
C. √4 = √1 + √3
2=1+√3 => NO
D. 2/4 = 2/1 + 2/3
1/2 = 8/3 => NO
E. -3(4) = -3(1) + -3(3)
-12 = -12 => YES
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RonPurewal
 Post subject: Re: EIVs - Formulas Question
 Post Posted: Tue Jul 21, 2009 7:44 pm 
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ManhattanGMAT Staff


Posts: 7146
for-which-of-the-following-functions-is-f-a-b-f-a-f-b-t2795.html
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