1, the elements of the set are deterministic, disordered and different from each other.

2. When dealing with condoms, we must pay attention to the "extreme" situation: or; When looking for a subset of a set, did you notice that it is a subset of any set and a proper subset of any nonempty set?

3. The key to judge whether a proposition is true or false is "grasping related words"; Note: "No" or "No" means "and" No "and" means "or".

4. The true and false feature of "or proposition" is "one truth, all false"; The true and false characteristics of the "and proposition" are "if one is false, it must be true"; The true and false feature of "non-proposition" is "one truth and one falsehood".

5. Among the four propositions, "negative exchanger", "negative negator" and the original proposition are equivalent to negative propositions, but the original proposition is not equivalent to negative propositions and negative propositions. The reduction to absurdity is divided into three steps: hypothesis, pushing spear, achievement and necessary and sufficient conditions.