... f(x1, x 2 ) = x1x 2 + x1x 2 + x1x 2 = (x1 + x1)x 2 + x1x 2 = x 2 + x1x 2 = x 2 + x1 Vê dủ: f(x1, x 2 , x3) = x1x 2 x3 + x1x 2 x3 + x1x 2 x3 + x1x 2 x3 ... x1x 2 x3 = x1x 2 x3 + x1x 2 x3 + x1x 2 x3 + x1x 2 (x3 + x3) = x1x 2 x3 + x1x 2 (x3 + x3) + x1x 2 = x1x 2 x3 + x1(x 2 + x 2 ) = x1x 2 x3 ... f(0, x 2 ). x1 + f(1,x 2 ).x1maỡ: f(0, x 2 ) = f(0,0 ). x 2 + f(0,1).x 2 vaỡ: f(1, x 2 ) = f(1,0). x 2 + f(1,1). x 2 Suy ra: f(x1, x 2 ) = f(0,0) x1x 2 + f(0, 1) x1x 2 + f(1,0...