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by alex.gellatly » Fri Apr 13, 2012 10:26 pm
For which of the following functions is f(a + b) = f(a) + f(b)for all positive numbers a and b ?
(A) f(x) = x2
(B) f(x) = x+1
(C) f(x) = x
(D) f(x) = (2) / (x)
(E) f(x) = -3x
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by niketdoshi123 » Fri Apr 13, 2012 10:53 pm
alex.gellatly wrote:For which of the following functions is f(a + b) = f(a) + f(b)for all positive numbers a and b ?
(A) f(x) = x2
(B) f(x) = x+1
(C) f(x) = x
(D) f(x) = (2) / (x)
(E) f(x) = -3x
assuming a=3 and b=5

a) f(x)=x2
f(a+b)= f(3+5)=64
f(a)+f(b)= f(3)+f(5)= 34

b)f(x) = x+1
f(a+b) = f(3+5)= 9
f(a)+f(b)= f(3)+f(5)=(3+1)+(5+1)=10

c)f(x)=x
f(a+b)=f(3+5)=8
f(a)+f(b)=3+5=8

d)f(x)=2/x
f(a+b)=f(3+5)=2/8=1/4
f(a)+f(b)=f(3)+f(5)= (2/3) + (2/5)=16/15

e)f(x)=-3x
f(a+b)=f(3+5)=-24
f(a)+f(b)=f(3)+f(5)= -9-15= -24

both [spoiler]c & e[/spoiler] satisfy the function
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by sam2304 » Fri Apr 13, 2012 11:29 pm
alex.gellatly wrote:For which of the following functions is f(a + b) = f(a) + f(b)for all positive numbers a and b ?
(A) f(x) = x2
(B) f(x) = x+1
(C) f(x) = x
(D) f(x) = (2) / (x)
(E) f(x) = -3x
Is the question proper ? ? Two right answers are there.
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by killer1387 » Fri Apr 13, 2012 11:42 pm
For which of the following functions is f(a + b) = f(a) + f(b)for all positive numbers a and b ?
(A) f(x) = x2
(B) f(x) = x+1
(C) f(x) = x
(D) f(x) = (2) / (x)
(E) f(x) = -3x

BOTH C and E are good here (doubt if its a GMAT PREP problem)

because both presents option that is a straight line passing through origin and will satisfy the function given in the question.
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by GMATGuruNY » Sat Apr 14, 2012 5:08 am
I posted a solution here:

https://www.beatthegmat.com/gmat-prep-fu ... 67229.html

Answer choice C should read √x.
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by alex.gellatly » Sun Apr 15, 2012 12:51 am
Sorry everyone for typing the answer choices incorrectly!!!!!! and Thanks a lot for your help.
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