GMAT Prep Question (functions)

[gxj007]

Can anyone provide some insight on this problem as well? thanks

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 of x
d) f(x) = 2/x
e) f(x) = -3X


Answer:
e) -3x

[arbiter]

Can anyone provide some insight on this problem as well? thanks

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 of x
d) f(x) = 2/x
e) f(x) = -3X


Answer:
e) -3x

f(a)=-3(a)
f(b)=-3(b)
f(a+b)=-3(a+b) --jus replace x values
= (-3a )+ (-3b)

ans E