Question from mba.com Practice Test

[dashlane]

I can't seem to answer the following question. May be extremely easy, but I'm missing something?

For which of the following functions is f(a+b) = f(a) + f(b) for all positive numbers a and b? The answer is e), f(x) = -3x

a) f(x) = x * x
b) f(x) = x+1
c) f(x) = square root of x
d) f(x) = 2/x
e) f(x) = -3x

[StaceyKoprince]

Please make sure to read (and follow!) the forum guidelines. GMATPrep math problems should be posted in the GMATPrep Math folder. I'll move it over there for you this time, but remember this for future, because it will take longer to get your questions answered if they're not posted in the appropriate folder! :)