Teaching material analysis: This lesson is arranged on the basis that students have mastered addition and subtraction and mixed operations of addition and subtraction. It can solve the practical problems of simple addition and subtraction and mixed operations of addition and subtraction, and provide a knowledge base for the study of brackets. At the same time, the appearance of brackets can also increase the changes of test questions, improve students' interest in practice and further master the operation order.
Teaching objectives:
1. Students understand the meaning and function of brackets, master the mixed operation sequence of addition and subtraction with brackets, and calculate the two-step problem of addition and subtraction with brackets.
2. Cultivate students' abilities of observation, comparison and generalization.
3. Stimulate students' interest in calculation through teaching and cultivate their good habit of examining questions.
Teaching emphases and difficulties:
1. Understand the function of brackets.
2. Master the calculation process and method of parenthesis addition and subtraction.
Teaching preparation:
Teacher: Multimedia Courseware
Student: Disc or stick, and so on.
teaching process
First, check the import.
1. Oral calculation (show card)
3+8= 4+5= 12-7=?
12+3= 13-5= 18-9=
7-4= 12+6= 1 1-8=
2. Tell the calculation process of the following questions.
4+3+3=? 5+0+3= 3+4+9=? 3+2+ 1=? 9-3-4= 10-3+7= 10+5-2= 7-0-6=
Second, teach new lessons.
1. Teaching Example 3. The scene diagram of Example 3 is given.
Teacher: Xiaoming and Xiao Yong help the teacher make teaching AIDS. * * There are 10 teaching AIDS. Cut off two first, then three. Can you help Xiao Ming calculate: How many are left? Students think independently, work out the surplus, and then communicate collectively.
Health 1: how many are left after cutting two, and how many are left after cutting three. The formula is: 10-2-3=5.
Blackboard: 10-2-3=5.
Teacher: In what order do you calculate? What's first? What is it?
Health 1: Count from left to right first. First, 10-2=8, then 8-3=5. Blackboard writing:
Teacher: Are there any other algorithms?
Health 2: You can also count how much you cut and how much is left.
Teacher: How should we work out this formula?
Health 2: 2+3 = 5 10-5 = 5.
Teacher: If these two formulas are combined into one formula, how should they be expressed? The group will discuss first, and then the whole class will communicate.
Students discuss activities in groups.
Report and communicate.
Health 3: 10-2+3 = 5
Teacher: Who will calculate this problem in order and see if the result is equal to 5?
Health 4: Calculate 10-2=8 first, then 8+3= 1 1.
Teacher: According to his algorithm, first calculate 2+3=5, and then 10-5=5. Now the formula is 10-2=8, and then 8+3= 1 1. They calculate in different order and the results are not equal. What should we do?
(Students don't understand)
Teacher: Today, the teacher invited a little guest for you. It can help us solve this problem. These are brackets. Show me the brackets.
Blackboard: brackets → (? )
Write two formulas, 2+3 = 5 and 10-5 = 5, into one formula. It is required to calculate 2+3 first, which can be expressed as follows:
Blackboard writing:
2. Understand the function of brackets.
Teacher: With brackets, we can change the operation order of the original formula. The function of brackets is to enclose the first part. In this question, we have to calculate 2 plus 3 first, and then put 2+3 in brackets.
Teacher: "10-(2+3)=" In this question, what comes first with brackets? What is it?
Health: Calculate 2+3=5 first, and then 10-5=5.
3. Summarize the operation sequence.
Teacher: Today we know parentheses, so what is the operation order of the two-step problem of parentheses? What's the difference between the order of operation and the two-step problem we learned before? Please talk to each other in the group.
Summary: There are parentheses in an expression, so the inside of parentheses should be counted first, and then the outside of parentheses should be counted.
Third, class assignments.
1. Complete "Doing" on page 74 of the textbook.
(1) Compare the similarities and differences between two corresponding problems.
(2) Tell me about their operation sequence.
(3) Independent calculation and class communication.
Fourth, class summary.
Teacher: What did you learn in this class? What did you get?
Students express their opinions and speak freely.
Summary: Today we met brackets. If you want to change the order of operations, you can calculate the latter first and put parentheses on the previous formula. Parentheses are very useful. As long as the children see them, they must first calculate the formulas inside.
Verb (abbreviation for verb) assignment
1. Exercise questions on pages 75-76 17, questions 5-10.
2. Complete the corresponding exercises of this lesson in the workbook.
Six, blackboard writing settings
In the fourth class, there are parentheses to do addition and subtraction.
There are parentheses in the expression, so you should count the parentheses first.