. dẫn: a) Đặt ϕ : X → R định bởi ϕ(x) = d (x, f(x )) , x ∈ X. Do: |ϕ(x) − ϕ(x )| = |d (x, f(x )) − d (x , f(x )) | d(x, x ) + d (f(x), f(x )) 2d(x,. n d(x, f(x )) + d(f(x), f 2 (x )) + · · · + d(f p−1 (x), f p (x )) k n (1 + k + · · · + k p−1 )d(x, f(x )) = k n 1 − k p 1 − k d(x, f(x )) Vậy d(x n ,...