Teaching purpose:
1. Through observation, analysis and comparison, guide students to master some simple addition and subtraction algorithms and understand arithmetic.
2. Develop students' observation and analysis ability and cultivate the flexibility of thinking. And effectively improve students' computing ability and cultivate students' good study habits.
Teaching emphasis: a simple algorithm for adding and subtracting a number close to integer ten or integer hundred.
Teaching difficulty: the calculation of a simple algorithm to subtract a number from an integer close to 100 or 10. (Add a few more if you subtract a few more)
[Comment: The appropriateness and concreteness of the teaching purpose, key points and difficulties show that teachers have an accurate grasp of the teaching materials and have a thorough understanding. ]
Teaching methods: heuristic method, migration method and infiltration method.
Learning methods: observation, comparison and induction.
Teaching tools: cards, projectors, etc.
Teaching process:
First, innovation.
(l) fill in the blanks.
78=80-( ) 87=( )-( )
99=( )-( ) 198=( )-( )
297=( )-( )
Question: Which integer is 78 close to? 78=80-( )? How much is 87 equal to dozens? How close is 99? How much is 100 minus? Imitate the first three questions and think about how to fill in the last two questions.
(2) Observe the two sets of formulas and think about which one is simpler? Why?
Group 1: 2 15+70 Group 2: 2 15+69.
143+ 100 143+98
475-200 475- 199
387-300 387-298
Inspire the students to answer, let them observe the second set of formulas and see what are the characteristics of addend and subtraction in this set. (Not an integer of ten or a hundred, but a close integer of ten or a hundred)
[Comment: Grasp the connection between old and new knowledge, carefully design the review content as an auxiliary buffer, slow down the tilt of knowledge, and lay a good foundation for students to learn new knowledge. ]
Teacher: When calculating addition and subtraction, if the addend or subtraction is a number close to integer 10 or integer 100, it is easier to calculate it as integer 10 or integer 100 first.
Teacher: This is "some simple addition and subtraction algorithms" that we are going to learn today. (blackboard writing topic)
Second, the new curriculum teaching
(1) Example 1: 1 13+59
(1) Find out which addend in the formula is a number close to integer ten or integer hundred. (59) (blackboard writing: looking for)
(2) Think about it, how many calculations can be counted as 59? After 60, how is the result of 1 13+60 better than the original question? Why 1? (Enlighten students to say that because there is an L)
Add 1, what should the original question be? The answer on the blackboard: add 1 and you will subtract L.
(3) After thinking about a simple calculation method, what should I do next? (blackboard writing: counting)
Please describe the simple calculation method completely.
1 13+59= 1 13+60- 1= 172
Tell the students that the intermediate step is to write the thinking process.
Summary: The problem we just did is actually carried out in three steps: search → thinking → calculation. "Find" is to find out which addend in the formula is close to the integer ten, and "Think" is a simple calculation method of thinking about it, (plus 1, minus 1). After thinking about the method, you can "calculate".
[Comment: Through this part of teaching, students are guided to discover → think → calculate, so that students can clearly understand the law, improve their calculation skills and cultivate their ability to solve practical problems in life through transformation methods. ]
(4) If 69 is added, how much should be considered? What about 79? What about 99?
Teacher: Let's learn a simple algorithm of addend close to integer hundred.
(b) Example 2: 276+98
(1) A simple algorithm for students to discuss this problem. Answer by roll call. 276+98=276+ 100-2=374
(2) Why should we add 100 and subtract 2? (Inspire the students to answer, if you add 2, you will subtract 2. Write on the blackboard. )
(3) How to change 98 into 97? Let the students try.
276+97=276+ 100-3=373
(4) Why subtract 3? The students answered, and the teacher wrote on the blackboard: (plus 3 MINUS 3)
[Comment: Let students try to calculate and give guidance on learning methods, highlighting the role of students as the main body and teachers as the leading factor, so that students can master learning methods while learning new knowledge and achieve the purpose of learning. ]
(3) Teacher's summary: How to calculate addend when it is close to integer 10 or integer 100?
Summarize the law of simple addition algorithm in one sentence: add more and subtract more (teacher's blackboard)
Simple calculations are carried out around three links: search → thinking → calculation. "Find" is to find out which addend in the formula is close to the integer ten or one hundred. "Thinking" means thinking about its simple calculation method: "add more and subtract more", and you can "calculate" when you think about it. Calculate two questions according to these three steps.
Try, think and fill in.
156+87= 156+90○□=□
74+ 198=74+200○□=□
(4) Can a simple algorithm be used for subtraction? (In fact, you can also follow the three steps of finding → thinking → calculating. )
Example 3 165-97
(1) Find out: Which number in this formula is close to whole ten or whole hundred?
How much calculation is easier? (97 as 100)
(2) Think about it: After 100, is it more or less? How much did you lose? Subtract 3, what can I do to get the original result? Teacher's blackboard: subtract 3 and add 3.
(3) Calculation:165-97 =165-100+3 = 68.
(4) If we subtract 2, how can we get the original result? What about negative 1?
Teacher's blackboard: subtract 2 from it and add 2, and subtract 1 and add 1.
(5) Think about it: the original title of 165- 100+ 1 should be 165- ().
(6) Summary: How to calculate the number whose subtraction is close to integer 10 or integer 100? Summarize the simple calculation law of subtraction in one sentence? Teacher's blackboard: add more if you subtract more.
Subtraction and simplification should also be carried out around three links: seeking → thinking-calculation.
(5) Reading questions
Complete "Done" on page 38.
Third, consolidate the practice.
(1) Look at the card and fill in the blanks.
For example:+198 as (plus 200 minus 2)
+88 as()
-99 as()
+297 as()
-297 as()
(2) Judge whether the following simple algorithm is correct. (judging by gestures)
A:127+59 =127+60-1
B:99+45=45+ 100- 1
C:243-98=243- 100-2
D:86+97=86+ 100-3
e: 12 1-89 = 12 1- 100- 1 1
(3) Choose the simplest algorithm.
A 86+89
( 1)86+80+9 (2)89+90-4 (3)86+90- 1
B: 198+84
( 1) 198+80+4 (2)84+200-2 (3) 198+90-6
C: 1 15-99
( 1) 1 15-90+9 (2) 1 15- 100- 1 (3) 1 15- 100+ 1
[Comment: The above three groups of exercises are lively and focused, achieving the goal of consolidation and strengthening. ]
(4) think about it and fill it in.
432-( )=432-200+2
376+( )=376+400-3
( )+277=277+ 100-4
522-( )=522-300+ll
[Comment: The design of this group of exercises skillfully uses the law of knowledge transfer and cultivates students' reverse thinking ability]
(5) Compare who found the simplest algorithm.
197+98 98+299
[Comment: The design of this group of exercises sublimates the teaching content of this section and creates opportunities for students with spare capacity to play their talents. ]
Fourth, summary.
What is the main content of our study today? When calculating addition and subtraction, if the addend or subtraction is a number close to integer 10 or integer 100, how can the calculation be simpler? What is the rule of simple addition calculation? What is the law of simple calculation of subtraction? It should be carried out around several links.
[Comment: After learning this section, in order to give students a complete and deep impression, the teacher systematically summarizes the knowledge learned by asking and answering questions]
[General Comment: The teaching purpose is clear, the thinking is clear and the connection is close. In knowledge teaching, we should recalculate the theory, grasp the process, fully mobilize students' enthusiasm, pay attention to giving full play to students' main role, and guide students to solve problems according to the method of "discovery, thinking and calculation". Students not only learned knowledge, but also learned the methods to solve problems, and the teaching effect was remarkable. ]