1. collocation is arranged on the basis that the second-year students have been exposed to the knowledge of permutation and combination. Permutation and combination is the primary knowledge of combinatorial mathematics, the basis of learning probability and statistics, and also the widely used mathematical knowledge in daily life.

Second, the teaching design focuses on infiltrating these mathematical ideas into students, initially cultivating students' awareness of thinking about problems in an orderly and comprehensive way, so as to implement the requirements put forward in the standard: "Students can think simply and methodically in the process of solving problems."

Third, in the whole teaching design, first of all, through "matching clothes" to create a situation, leading to the problem of matching, so as to understand the mathematical thought of matching. Then, let the students go through the whole mathematical process of "guessing, posing, speaking, drawing and calculating", solve the problem of "matching three coats and two trousers", infiltrate the combination idea and develop the sense of symbol.

Fourthly, in the process of exploring the arrangement problem, it is mainly to cultivate students' awareness of orderly thinking. Through independent completion and group cooperation and communication, students' mathematical thinking is triggered, and they are arranged in an orderly and disorderly manner, so that students can realize the benefits of orderly thinking; No repetition, no omission. In the face of new textbooks, we should deal with them appropriately and flexibly on the basis of studying the content and writing intention of the textbooks.

5. Organize students to operate with cards or numbers or symbols instead of objects. On the basis of this activity, the students discovered "connection method" and "calculation method". It can be seen that in teaching activities, designing activities according to the teaching purpose, allowing students to operate intuitively and asking questions at the same time, will guide students to think deeper and deeper, and there will be unexpected gains.