Key points: rewriting and abbreviation of large numbers.
Difficulty: Rewrite the number of non-integer ten thousand into the divisor of "ten thousand".
Teaching aids: multimedia courseware, slides.
Teaching process:
First of all, old knowledge paved the way.
Show the following questions with slides.
1, write the following numbers
1,202,304,503,206,858,073,648,000.
2. Read the numbers below.
2000000 3500000 10200000 86000000
Second, teach new lessons.
1, teaching example 5.
(1) Display wall charts or computer courseware to show the structure of human blood and introduce the functions of red blood cells and white blood cells.
Explain how many red cells and white cells are contained in a drop of blood. The teacher showed the blackboard.
A small drop of blood contains: red blood cells: 5000000 white blood cells: 10000.
(2) Let the students read these two numbers. Let's take a look at the characteristics of these two numbers.
After the students talk, the teacher explains that these two numbers are integers. What is a full ten thousand?
For example, 2000035000010000001020000 ... are all integers.
Likes: 2003535600010000051025600 ... all are non-integers.
(3) Tell students clearly that in production and life, in order to facilitate reading and writing, people often omit the four zeros behind 10,000 and replace them with a word "10,000". Replace the four zeros at the end of the number with ten thousand words, so that the whole ten thousand numbers are rewritten into a number in units of ten thousand.
(4) rewriting.
Blackboard: 500 | 0000 = 5 million.
Key point: omit the four zeros of each level and write the word "ten thousand" after 500. So it should be read like this: 5 million.
Think about it: 10000 = 10000. Let the students talk first, and then the teacher adds "1" to the blackboard.
(5) Try it immediately.
Rewrite the following numbers into numbers in units of "ten thousand". 180000 300000 18000000
Students rewrite it themselves. The teacher patrolled to observe whether the students remembered to write the word "10 thousand" as a key emphasis.
2. Teaching example 6.
(1) Computer courseware shows maps of the sun and the earth, allowing students to observe and collect news and understand the relationship between the sun and the earth.
(2) Ask the students to read the diameters of the sun and the earth, and tell how many times the diameter of the sun is that of the earth. At this time, it is difficult for students to directly tell the multiple relationship between these two large numbers.
(3) Inspire students to think that it is more convenient to compare numbers with "ten thousand".
(4) What are the characteristics of these two numbers?
By observing the students, it is found that these two numbers are both non-integer tens of thousands. Obviously, we can't directly write them as "tens of thousands" as in Example 5.
So what should we do?
(5) Introduce the rounding method and how to rewrite a non-integer into a number with "ten thousand" as the unit.
Teacher: There is a way to find the divisor called "rounding". What is "rounding"? For example, 12756 is rewritten as a number in units of "ten thousand" by rounding.
(1) Find the number of ten thousand. Blackboard: 12756
(2) Look at the first number on the right of ten thousand places, which is 2.
③ Description: According to the method of "rounding", if it is less than 5 like this, discard it and the number on the right and rewrite it as 0.
Blackboard: 12756≈ 10000.
Note: Because the approximate value is obtained, "=" must be replaced by "≈".
④ Rewrite 10000 into a number with "ten thousand" as the unit.
Blackboard 10000 = 10000 (note: here the two numbers are equal, only "=" can be used).
Think about it: in this way, 1389000 is rewritten into a divisor with the unit of "ten thousand" by rounding.
Let the students try to practice first, and then the teacher will guide them step by step.
① Find ten thousand digits: 1389000.
② Look at the number at 1 to the right of the ten thousand digits. If it is greater than 5, go forward 1, then discard it and the number on the right and rewrite it as 0.
Blackboard: 1389000≈ 1390000
③ Rewrite 1390000 on the blackboard into a number with "ten thousand" as the unit.
Blackboard: 1390000 = 139000.
Teacher: How many times the diameter of the sun is that of the earth?
Once, try immediately.
Rewrite the following figures into "ten thousand" figures by rounding. 1264008 1328543 1209000
First, students try to practice, and then the teacher comments. It's ok for students to "sketch" figures directly in units of "ten thousand", but "material" should be emphasized.
Such as:1264008 ≈126001328543 ≈133000.
Third, classroom activities.
1, text page 14 "Do it".
Through practice, on the one hand, let students rewrite what they have just learned and further master new knowledge; On the one hand, students can learn about China's geographical knowledge and broaden their horizons by providing relevant geographical knowledge materials.
2. "Do it" on page 15 of the text
By asking students to find different divisors of the same large number and practicing "rounding" to find divisors, students can deepen their understanding of divisors.
Fourth, consolidate practice.
Questions 3-8 in exercise 2 of the text.