When calculating the collocation problem, collocation in a certain order is to highlight and fix an object first, and collocate it in an orderly way, so as not to repeat or omit. Number of collocation methods = product of numbers. The formula of handshake problem is: the number of handshakes = number of people × (number of people-1)÷2.

1: Wang Li has two tops and three bottoms, so how many different ways does Wang Li have to match a suit?

Analysis: According to collocation method, the number of species = the product of numbers. You can see that there are two tops and three bottoms.

Solution: 2× 3 = 6 (species)

Wang Li has six different ways to make a suit.

Lecture 2: There are five roads from A to B and two roads from B to C ... How many different ways are there for a car to go from A to B?

Analysis: A to C is divided into two steps. Step one, A to B, there are five roads. Step two, from B to C, there are two roads. Using the principle of multiplication, we can know that there are 5× 2 = 10 methods from A to C * * *.

Solution: 5× 2 = 10 (species)

A: There are 10 different moves.

Lecture 3: At a party, 10 students shake hands with each other. How many times does a * * * shake hands?

Analysis: Suppose five people, ABCDE, A and A, want to shake hands with the other four people, then A shakes hands four times. Similarly, the other four people shake hands four times, so the total number of times they shake hands is 4×5=20. But pay special attention to the fact that A and B shake hands with you at the same time, which is the same. So the total number of times should be divided by 2, that is, 20÷2= 10 times. Based on this reasoning, suppose there are n people and the number of handshakes is (n- 1)×n÷2 times.

Solution:10× (10-1) ÷ 2 = 45 (times)

A: You have to shake hands 45 times at a time.

Methods of learning primary school mathematics well

1, lay a good foundation: primary school mathematics is the foundation of follow-up study, so we should pay attention to mastering basic mathematical concepts and operation skills, such as addition, subtraction, multiplication and division, fractions, decimals, fractions, etc.

2. Do more exercises: Mathematics is a subject that needs constant practice. Through a lot of practice, it can help to consolidate knowledge and cultivate logical thinking and problem-solving ability.

3. Systematic learning: Mathematical knowledge has a clear hierarchy and logic, and systematic learning should be carried out in the order of textbooks. At the same time, teaching AIDS, learning videos and other auxiliary tools are also powerful help to understand and master knowledge.