On the basis of students' preliminary understanding of fractions and decimals, this unit allows students to systematically learn the meaning and nature of decimals, pay attention to students' understanding of the meaning of decimals in the arrangement, and develop students' sense of numbers. Compared with the experimental textbook, the changes in this unit are as follows:
1, pay attention to the understanding of decimal meaning.
The textbook downplays the reason why decimal fraction can be expressed in decimal according to integer writing, and focuses on explaining the meaning of decimal from "decimal is another expression of decimal fraction" In the specific arrangement, the decimal relationship of measurement units (such as length units) is mainly used to help students understand.
2. The expression of "the change law of decimal size caused by decimal position movement" is improved.
3. Highlight the refinement of laws and regulations.
This unit involves many rules and regulations, such as the reading and writing methods of decimals, the nature of decimals, the method of comparing decimal sizes, the law of decimal size changes caused by decimal movement, the conversion of decimal units, and the method of finding decimal approximate numbers.
Extended data:
Properties of decimals
1, add or delete any zeros at the end of the decimal part, and the size of the decimal remains the same. For example: 0.4=0.400, 0.060=0.06.
2. Move the decimal point to the right (or left) by n digits respectively, and the value of the decimal point will be expanded (or reduced) by n times.
Conversion between decimals and fractions
1, limited score: reduced to a few tenths (a few percent ...).
2. Decimal fraction of pure cycle: the cycle segment is a numerator, and if there is a cycle segment, the denominator is 9; The loop segment has two digits, and the denominator is 99; The loop has three digits, the denominator is 999, and so on.
3. Mixed cyclic decimals: simplification after reduction to the sum of finite decimals and pure cyclic decimals.
4. Infinitely circulating decimals are irrational numbers and cannot be converted into fractions.
People's Education Press-Introduction to the second volume of fourth-grade mathematics in compulsory education textbooks
Baidu encyclopedia-decimal