Let f be an analytic function on the unit dist D = {z ||z| < 1} and suppose that there is a constant

 Let f be an analytic function on the unit dist D = {z ∈ Ȼ ||z| < 1} and suppose that there is a constant M ≥ 0 such that |f’| ≤ M on all of D. Prove that

for all z ₁ , z ₂ ∈ D.  

f(21) f(2)M21-2