... tc 1 (tng các tích s).Xét f(x) = x:Ta có: x =0.x + 1.xt khác: ( ) () ( )==⇒=00f11fxxfSuy ra: f(x) = x có th biu din:f(x) = x = f(0).x + f (1 ).xtrong ó: f (0 ), f (1 ) ... f(x) =x :Ta có: x = 1. x + 0. xt khác: ( ) () ( )==⇒=10f01fxxfSuy ra: f(x) =x có th biu din:f(x) =x = f(0). x + f(1).xXét f(x) = α ( là hng s):Ta có: α = α.1 = α.(x ... hàm 1 bin, ta có:f(0,x2) = f(0,0).x2 + f(0,1).x2 f(1,x2) = f(1,0).x2 + f(1,1).x2Suy ra:f(x1,x2) = f(0,0).x1x2 + f(0,1).x1x2 + f(1,0).x1x2 + f(1,1).x1x2ây...