2. For which of the following function f is f(x) =f(1-x) for all x.
1.f(x)=1-x
2. f(x)=1-x^2
3.f(x)=x^2-(1-x)^2
4. f(x)=x^2(1-x)^2
5. f(x)=x/(1-x)


Answer is 4 -- can anyone explain pls?