GMATPrep - Difficult Functions Question

[tutonaranjo]

Can someone explain this in detail? No idea how these function questions work.

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) = sqrt x
d) f(x) = 2/x
e) f(x) = -3x

Answer is E

[speed]

put a=1 , b=1

for choice a) f(x) = x^2 ; f(a) = 1. f(b) =1, f(a+b) = (1+1)^2=2^2 no match

similarly other choices too have LHS != RHS EXCEPT CHOICE E

ANSWER IS (E)

[kajcha]

IMO, the best approach to solve such problems is to substitute numbers.