For i), note that z3-z2 is vector BC, and z3 is vector OC. As the radius is perpendicular to the tangent, angle OCB will be pi/2. Thus, (z3-z2)/z3 is imaginary, and similarly so is (z2-z1)/z2.
ii) This is just tangents from a point have equal lengths.
iii) Here, I would prove that OB and AC are...