Function f(n)

Nigogo wrote:GMAT GURUs! Please help me:)
The function f is defined for all positive integers n by the following rule: f(n) is the number of positive integers each of which is less than n and has no positive factor in common with n other than 1. If p is any prime number then f(p)=
A. p-1
B. p-2
C. (p+1)/2
D. (p-1)/2
E. 2

Nigogo wrote:Here is another one thank u in advance!
For which of the following functions is f(a+b) = f(a) + f(b) for all positive numbers a and b?
f(x)=x2
f(x)=x+1
f(x)=√x
f(x)=2/x
f(x)=-3x