1. Theorem for determining the parallelism of two planes: If two intersecting lines in one plane are parallel to the other plane, then the two planes are parallel.

2. Recursive formula: the 1 item (or the previous items) of the sequence {an} is known, and any item an and its previous item an- 1 (or the previous items) can be expressed by a formula, which is the recursive formula of the sequence.

3. Theorem for judging out-of-plane straight lines: A straight line between a point in the plane and a point out of the plane and a straight line in the plane that does not pass through the point are out-of-plane straight lines.

4. An empty set is a subset of any set, and an empty set is a proper subset of any non-empty set.

5. The front view reflects the up, down, left and right position of the object, that is, it reflects the height and length of the object; The top view reflects the position relationship between the left and right of the object, that is, the length and width of the object; The side view reflects the up-and-down and front-and-back positional relationship of the object, that is, it reflects the height and width of the object.