If f(x+y)=f(x)+f(y)f(x+y) = f(x) + f(y)f(x+y)=f(x)+f(y) for all x,yx, yx,y and f(1)=5f(1) = 5f(1)=5, find f(n)f(n)f(n) for any integer nnn.
5n5n5n
n5n^5n5
5n5^n5n
n+5n+5n+5