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Find the approximate number (a)
Wu Jiong Road and Lin Qili of Hu Ming Primary School in Xiamen, Fujian.
Teaching content:
Textbook page 14- 15 Examples 5 and 6, "Doing" and Exercise 2, Questions 3-5 and 7-8.
Teaching purpose:
The integer of 1 .10,000 will be changed to a number in units of 10,000.
2. I will use the method of "rounding" to omit the mantissa after tens of thousands within 100 million and find its approximate number.
3. Guide students to observe and experience the close relationship between mathematics and life, make students realize that mathematics knowledge comes from life and serves life, and cultivate students' spirit of active inquiry and consciousness of using mathematics.
Teaching emphasis, difficulty and key:
1. Important: An integer of 10,000 can be rewritten as a number in units of "10,000".
2. Difficulties: You can correctly omit the mantissa after 10,000 and write its approximate number.
3. Key: Bring some scenes in life to students, from cause to effect, and let students experience the practical application of "approximation" in social life.
Teaching process:
First of all, the teaching rewrites the integer of 10 thousand into a number with 10 thousand as the unit.
1. Projection shows pictures of white blood cells and red blood cells, and introduces white blood cells: it can eliminate germs and clean blood; Red blood cells: can transport oxygen. A drop of blood contains 5 million red blood cells and 0/0000 white blood cells.
2. Ask the students to read the number of red blood cells and white blood cells.
(1) According to the four-digit classification, the above three numbers are expressed in the following form:
500 0000 1 0000
Let the students read two numbers: five million and ten thousand.
Teacher: What do you find after reading these figures?
(4) Teachers write the following contents on the blackboard according to the students' reading process:
500 0000 = 5 million 1 0000 = 1 10,000
3. Students observe and compare the numbers on the right and left of the equal sign.
Students, look carefully. What's the difference between the number on the right of the equal sign and the number on the left of the equal sign?
(The number on the right of the equal sign omits the mantissa after ten thousand digits, and the number on the left of the equal sign does not omit the mantissa after ten thousand digits.
② What do they have in common? (The numbers on both sides of the equal sign are exactly the same size)
4. Students discuss in groups:
Let the students think about how to use "ten thousand" as the unit to represent the integer of ten thousand. To represent an integer in ten thousand units, just remove the four zeros after the ten thousand digits and write the word "ten thousand". )
(2) What are the advantages of expressing numbers in tens of thousands?
It is simple to express numbers in units of ten thousand, and it is not easy to make mistakes, so that people can know the size of numbers at a glance. )
5. Summary: In order to facilitate reading and writing, you can directly use "ten thousand" as the unit to represent integers in the future.
Exercise:
(1) Let the students finish the "Doing" questions on page 14 1 and 2, and the teacher will patrol.
(2) After rewriting, select some students to show the completed exercises on the display platform for collective evaluation.
Second, the teaching uses the "rounding" method to find the divisor.
1. Import:
Some big numbers are sometimes unnecessary or impossible to tell their exact numbers. For example, in Chongqing, there are about 15000 people who take part in long-distance running, and this 15000 people is an approximate number. For example, Beijing's bid to host the 2008 Olympic Games is US$ 2,000,000 (20 million), which is about RMB160 million. This1600,000 is just a rough figure. Since so many divisors are used in life, we should pay attention to the study of divisors. How to approximate a number?
We have learned to find an approximation of a number by rounding.
2. Review:
How to omit the mantissa after 4926 and 9375 thousand digits? What's the difference between two numbers omitted? Guide the students to say how to "round" the mantissa after omitting thousands of digits according to the figures on hundreds. )
Teacher: If you enlarge the number to a number greater than 10000, can you find its divisor in the same way?
The teacher gave an example.
(1) Let the students try to do it and assign a student to finish it on the blackboard.
(2) Revised version, and then discuss in groups: (1) Which number should be "rounded" when the mantissa after 12756 and 1389000 is omitted? (2) When calculating the divisor, the number of thousands 12756 is less than 5. What should I do? What if the number on 1389000 is greater than 5? (3) Why not use "equal sign" instead of "approximate equal sign" for the divisor?
③ Guide students to solve the above three problems through discussion. Special attention should be paid to make students understand that "approximate equal sign" should be used because it is an approximation of a number, not an exact number.
(4) Ask the students to complete the topic of "doing" on page 15, and then ask the students what they think.
4. Summary:
Students, we have learned to omit the mantissa after 10 thousand digits from a larger number and find the divisor; We also learned to rewrite an integer of 10 thousand into a number in units of 10 thousand. What are the similarities and differences in meaning and methods between these two aspects?
② Students discuss in groups, and then each group recommends a representative to report the discussion results. Finally, the teacher concluded that both seeking divisor and rewriting number need to change the expression form of number, but the essence is different. Finding the approximate number changes the size of the original number, while taking "ten thousand" as the unit only changes the expression form of the number, but does not change the size of the number.
Third, consolidate the practice.
① Complete Exercise 2, Questions 3 and 5.
When revising, let the students talk about the difference between rewriting it into a number with "ten thousand" as the unit and calculating the divisor by omitting the mantissa after ten thousand.
② Students independently complete Exercise 2, Question 4.
Fourth, class summary.
Teacher: Students, please recall this lesson.