I think you are misunderstanding. Z is set equal to zero before we do any interations. This is referred to as a "starting condition". So we set Z = 0 right at the beginning, then we square it (Z x Z = 0 x 0 = 0), and then we add C. Now Z = C. Since the first iteration always sets Z = C, why not just set Z = C right as the starting condition. That's one less iteration we have to do. This may seem like a small point but it is actually an important point. I will explain more in future posts.