"Combinatorial mathematics" counting principle

Counting principle

① Drawer principle

There are N drawers, n+1 apples, so at least one drawer has two or two apples.

There are N drawers and N-1 apples, so at least one drawer has no apples.

② addition principle

If Category A items have a, B items have B, then category A items or Category B items have a total of a+b (no case of the same nature)

③ Multiplication Principle

If A has a mode of occurrence in a, B has a mode in B, then the occurrence of events A and B has a*b in the way.

④ principle of tolerance and repulsion

∪= and A∪b writing a∪b, read A and B:

∩= a∩b writing a∩b, read as A cross B:

DeMorgan theorem: Set A and B as any two subsets of the complete U, then

Emoticons

, A's complement is the yellow part and the blue part

The complement of B is the red part and the blue part

The complement of A∩B is three parts of red, yellow and blue

Emoticons

, A's complement is the yellow part and the blue part

The complement of B is the red part and the blue part

The complement of A∩B is three parts of red, yellow and blue

"Combinatorial mathematics" counting principle