f (2x) = f (x + y) f (x-y) for all x, and belongs to R

f (2x) = f (x + y) f (xy) for all x, and it belongs to R & f (10) = 4, f (0)! = 0 and f & # 39; (0) = 2, then the value of f & # 39; (10) is?