The function f (x) defined on R, for any x, y ∈ R, f (x + y) + F (X-Y) = 2F (x) f (y), and f (0) is not equal to 0, it is proved that f (x) is even function

The function f (x) defined on R, for any x, y ∈ R, f (x + y) + F (X-Y) = 2F (x) f (y), and f (0) is not equal to 0, it is proved that f (x) is even function

Let y = 0
Then f (x) + F (x) = 2F (x) f (0)
F (0) is not equal to 0
We get f (0) = 1
Let x = 0 again
Then f (y) + F (- y) = 2F (0) f (y)
We obtain f (y) = f (- y)
So f (x) is even function