I. Knowledge and skills:
1. Guide students to explore and understand the laws of multiplication, exchange, association and distribution, and use algorithms to do some simple calculations.
2. Cultivate students' awareness and ability to choose algorithms according to specific conditions, and develop the flexibility of thinking.
3. Let students feel the connection between mathematics and real life, and use what they have learned to solve simple practical problems. ?
Second, the process and method:
1, through the discovery process of multiplicative commutative law, associative law and distributive law, experience observation and comparison, demonstrate with examples, and summarize learning methods.
2. Experience the connection between mathematical knowledge and its wide application through the application process of multiplicative commutative law and associative law. ?
Third, emotions, attitudes and values:
Let students feel the joy of discovering knowledge, stimulate students' interest and feel the connection between mathematics and life. Cultivate students' interest in learning and using mathematics.
Fourth, the lecture content:
Students, environmental protection is very important to human beings. We always try our best to protect the earth and the environment. Planting trees is a very meaningful thing. Has everyone participated in tree planting activities? Look at Xiaoming's classmates. They are planting trees. Let's go and have a look.
Students take part in tree planting activities. There are 25 groups. Four people in each group are responsible for digging holes and planting trees, and two people are responsible for carrying water and watering trees. How many people are in charge of digging holes and planting trees?
According to the known situation, there are 25 groups with 4 people in each group, who are responsible for digging holes and planting trees. Find out how many people are responsible for digging holes and planting trees, that is, how many people are 4 or 4×25, that is, how many people are 4 of 25, and multiply them to calculate: 25×4.
Problem solving: 25×4= 100 (person) or 4×25= 100 (person)
Observe the formula, and find the law: 4= 100 (person) or 4×25= 100 (person). We find that the factors of the two multiplication formulas are the same, and the exchange factor is 25×4=4×25. Therefore, it can be concluded that the position of two factors is exchanged in 25×4 and the product is unchanged, which is called multiplicative commutative law.
The law is expressed in letters: b=b×a(a, b stands for any number). If a and b are used to represent any factor, it is more intuitive and convenient to use letters to represent it.
On the blackboard: the law of multiplication and exchange A× B = B× A. Summarize the position of multiplying two numbers and exchanging two factors, and the product is unchanged. This is the so-called multiplication exchange law.