... = x (1, 1) f ( + h ,1 + k ) = f ( 1, 1) + f 'x ( 1, 1) h + f 'y ( 1, 1) k + f ''xx ( 1, 1) h + 2f ''xy ( 1, 1) hk + f '' yy 2! Ta có : f ( 1, 1) = f 'x ( x , y ) = yx y 1 ⇒ f 'x ( 1, 1) = ... ý ( x1, y1 ) ∈ Bδ ( x o ,y o ) , đặt z1 = z ( x1, y1 ) Với ( x, y ) ∈ Bδ ( xo ,yo ) ta có = F ( x1 , y1 , z1 ) − F ( x , y, z ) = F ( x1 , y1 , z1 ) − F ( x1 , y1 , z ) + F ( x1 , y1 , z ... = ( z − z1 ) F 'z ( x1 , y1 , θ ) + F ( x1 , y1 , z ) − F ( x , y, z ) (θ ∈ I) ⇒ z − z1 = F ( x , y, z ) − F ( x1, y1, z ) F 'z ( x1 , y1 , θ ) ≤ F ( x , y, z ) − F ( x1, y1, z ) β Cho...