functions problem

Posted by tkt on May 01, 2013; 2:06pm
URL: http://discussion-forum.276.s1.nabble.com/functions-problem-tp7580283.html

IF f:R->R, such that f(x+y)+f(x-y)=2f(x)f(y) and f(0) is not equal to 0, then f(x) is :

(A) even function
(B) odd function
(C) periodic function
(D) none

pls help me...