( ( )) ( ( )) ( ( )), ,F XYF X  F XF Y  X F Y F X X YR

Câu 3: (1 điểm) Tìm

f R: R

thỏa mãn:

(

( ))

(

( ))

(

( )),

,

f x

yf x

f xf y

 

x

f y

f x

x y

R

.