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brcinsf
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PS- GMAT Prep 21 Jun 2006, 22:36
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I can't wrap my head around this one. Please help.



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quangviet512
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PS- GMAT Prep 22 Jun 2006, 04:06
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We have f(x)=-3x
so f(a)+f(b)=-3a-3b=-3*(a+b)=f(a+b)
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PS- GMAT Prep 22 Jun 2006, 04:13
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f(a+b)=f(a)+f(b) is true only for multiplication
therefore only the last is true f(x)=-3x
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brcinsf
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PS- GMAT Prep 22 Jun 2006, 09:57
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duh! :wall Thank you folks!

-Tru
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haas_mba07
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PS- GMAT Prep 22 Jun 2006, 10:03
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Clearly E.

-3(a+b) = -3a + (-3b)
f(a+b) = f(a) + f(b)
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paddyboy
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PS- GMAT Prep 22 Jun 2006, 16:04
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Yup. E.

Hey, if you don't mind my asking, how much did you score on that test? Just curious...
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MA
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PS- GMAT Prep 22 Jun 2006, 18:01
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guys, i am dumb on this one. i like to know every step on this one.
thankx..
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haas_mba07
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PS- GMAT Prep 22 Jun 2006, 18:32
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For given functions A..E we need one which is such that
f(a+b) = f(a) + f(b)

A. f(x) = x^2 i.e. f(a+b) = (a+b)^2
This is not equal to a^2 + b^2 = f(a) + f(b)

B. f(x) = x+1
f(a+b) = a+b+1
f(a) + f(b) = a+1 + b+1 = a+b+2 which is not eq. f(a+b)

C. f(x) = sqrt(x)
f(a+b) = sqrt(a+b) which is not eq. sqrt(a) + sqrt(b)

D. f(x) = 2/x
f(a+b) = 2/(a+b) which is not eq to 2/a + 2/b

E. f(x) = -3x
f(a+b) = -3(a+b) = -3a + (-3b) = f(a) + f(b). Equal.

Therefore E.

MA
guys, i am dumb on this one. i like to know every step on this one.
thankx..
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MA
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PS- GMAT Prep 22 Jun 2006, 19:18
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great. you guys are ginious.

now its pretty clear. thanx a ton again.

haas_mba07
For given functions A..E we need one which is such that
f(a+b) = f(a) + f(b)

A. f(x) = x^2 i.e. f(a+b) = (a+b)^2
This is not equal to a^2 + b^2 = f(a) + f(b)

B. f(x) = x+1
f(a+b) = a+b+1
f(a) + f(b) = a+1 + b+1 = a+b+2 which is not eq. f(a+b)

C. f(x) = sqrt(x)
f(a+b) = sqrt(a+b) which is not eq. sqrt(a) + sqrt(b)

D. f(x) = 2/x
f(a+b) = 2/(a+b) which is not eq to 2/a + 2/b

E. f(x) = -3x
f(a+b) = -3(a+b) = -3a + (-3b) = f(a) + f(b). Equal.

Therefore E.
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