The two principles of addition and multiplication are the laws that run through. What has nothing to do with order is combination, but what needs order is arrangement.

Two formulas, two properties, two ideas and methods. Arrange and combine the summary, and the application questions must be transformed.

It is common sense to arrange and combine together and choose the back row first. Special elements and positions should be considered first.

Don't worry too much, and don't miss too much. Punching is a skill. Arrange combinatorial identities and define proof modeling tests.

On binomial theorem, China Yang Hui Triangle. Two properties, two formulas, function assignment transformation.

Related introduction:

Although mathematics began in ancient times, there was no skill because the development of social production level was still in the low stage. With people's understanding and research on numbers, in the process of forming mathematical branches closely related to numbers, such as the formation and development of number theory, algebra, function theory and even functional, the diversity of numbers is gradually discovered from the diversity of numbers, and various counting skills are produced.

At the same time, people have a profound understanding and research on numbers. In the process of the formation and development of various mathematical branches closely related to shapes, such as geometry, topology and category theory, the diversity of numbers and shapes is gradually discovered from the diversity of shapes, and various skills of numbers and shapes are produced.

Modern set theory and mathematical logic reflect the potential combination of number and shape. However, modern algebraic topology and algebraic geometry closely link numbers with shapes. All these have had and will continue to have a far-reaching impact on the formation and development of modern combinatorics centered on digital skills.