Numbers like 0, 1, 2, 3, 4, 5, 6 are natural numbers.

2.

Things like -3, -2,-1, 0, 1, 2, 3, ... are integers.

Integers include natural numbers.

3. Multiples and factors: Multiples and factors are interdependent. For example: A×B=C, it can be said that A is a multiple of B and C, and B and C are factors of A, for example, 20 is a multiple of 4 and 5, and 4 and 5 are factors of 20. Note: We only study multiples and factors (except 0) within the range of natural numbers.

4. Odd and even numbers: numbers that are multiples of 2 are called even numbers, and numbers that are not multiples of 2 are called odd numbers.

5. Find a factor: find a factor of a number, one-on-one and orderly search, without repetition and omission. The smallest factor of a number is 1, and the largest factor is itself.

6. Multiply: from 1 time, search in order. A number has no maximum multiple. The smallest multiple is itself.

7. Prime number: A number has only two factors, 1 and itself. This number is called prime number.

8. Composite number: A number has other factors besides 1 and itself. This number is called a composite number. Note: 1 is neither a prime number nor a composite number.

9. According to the factor of a number, natural numbers can be divided into (prime number), (composite number) and (1 and 0).

. According to the parity of a number, natural numbers can be divided into two categories (odd and even). 0 is the smallest even number.

10. Supplement: Integer A is divisible by Integer B (B is not equal to 0), and the divisible quotient is exactly an integer without remainder, so we say that A can be divisible by B.

Multiplication characteristics of11.2,3,5: Numbers with 0,2,4,6,8 are multiples of 2. A number with 0 or 5 digits is a multiple of 5. The sum of each number is a multiple of 3, and this number is a multiple of 3.

12. prime factor: each composite number can be written as the product of several prime numbers. Every prime number is a factor of this composite number, which is called the prime factor of this composite number.

13. A composite number multiplied by a prime factor is called prime factor decomposition.

14. The common factor of several numbers is called the common factor of these numbers. The biggest one is called their greatest common divisor.

15. Two numbers whose common factor is only 1 are called prime numbers.

16. The common multiple of several numbers is called the common multiple of these numbers, and the smallest is called the least common multiple of these numbers.

17. A fraction whose denominator is a prime number is called the simplest fraction.

18. approximation: it is called approximation to change a fraction into a fraction equal to it, but with smaller numerator and denominator.

Note: Try to calculate with your mouth when making an appointment. Generally, the common factor of the numerator denominator (except 1) is used to remove the numerator denominator of the fraction; Usually, we have to separate it until we get the simplest score.

19. Comprehensive score: the score of different denominators is changed into the score of the same denominator equal to the original score, which is called comprehensive score.

The general method of general division is: first find the least common multiple of the original denominator, and then divide the score into fractions with this least common multiple as the denominator.

20. Decimal decimal, there are several decimals, just write a few zeros after 1 as the denominator, and remove the decimal point after the original decimal point as the numerator; After the number of components, the number of quotation points can be reduced.

2 1. Fractional decimals with denominators other than integer 10, integer 100 or integer 1000 shall be denominated, and several decimals may be reserved according to rounding if necessary.

22. (a simplest fraction, if the denominator does not contain other prime factors except 2 and 5, this fraction can be reduced to a finite decimal; If the denominator contains prime factors other than 2 and 5, this fraction cannot be reduced to a finite decimal. )