Question 6. Let (t E R. Prove that the cut plane D = (C \\ { z 2 re\" : r 2 0} is open. [5 marks] You may use the result of the previous question if you nd it useful to do so... Definition 1.2.2. A subset U of C is open if for all zo E U, there exists & > 0 (depending on zo) such that DE(Zo) C U. A subset F of C is closed if its complement C\\F is open. A neighbourhood of a point zo E C is an open set which contains zo