1, three characteristics of a set: certainty, as an element of a set, must be determinable. Mutual, for a given set, the elements in the set are different from each other. Out of order, the elements in the collection have no order.

2. Enumeration: When there are few elements in the collection, list the elements in the collection one by one and write them in braces to represent the collection.

3. Set theory: If there is a one-to-one correspondence between the elements of two infinite sets M and N, then they contain the same number of elements.

4. Common factor extraction method: the common factor is extracted and decomposed into the product of two linear factors, and the quadratic inequality of one variable is transformed into two linear inequalities for solution.

5. Divide a fillet into 360 equal parts, and stipulate that each equal part is an angle of 1 degree. This system of measuring angles in degrees is called an angle system. The radian system is a system for measuring angles in radians.