As Q Chris said 9 is clearly marked as a premise and so does not follow any other therefore one can not use 9 to disprove 1 2 or 3 since they have no logical inter dependence The fact that premise 9 creates a contradiction only proves that the argument assuming premise 9 is invalid It would be perfectly valid though incomplete assuming only 1 2 3 Again Reductio ad Absurdum arguments show that not all premises can be true not that all premises are not true