(1) _ _ _ _ _ _ _ are collectively called natural numbers.

(2) Among positive integers, _ _ _ (Yes, No) is the largest number. _ _ _ (Yes, No) The smallest number; It's _ _ _ _.

(3) 2.8 ÷ 2 = 1.4, (can, can't) say that 2 is divisible by 2.8.

(4) The factor of16 is _ _ _ _ _.

(5) Among positive integers within 20, multiples of 3 range from small to large.

(6) The parity of positive integers can be divided into odd and even numbers. If a > 3, a is odd, then the odd number adjacent to a is _ _.

(7) The number of _ _ _ is called prime number, and the number of _ _ _ is called composite number. Write all prime numbers within 20 _ _ _ _ _, and write all composite numbers within 20 _ _ _ _ _.

(8) Decomposition factor: 72 = _ _ _ _ _ _.

(9) The greatest common factor of16 and 24 is _ _ _ _ _ _ _. Common multiples of 2 and 3 within 20 are _ _ _ _ _ _ _ _.

(10) A pile of apples, more than 50 and less than 70, can be divided into two piles, three piles and five piles. There are _ _ _ _ apples in this pile, which can also be divided into _ _ _ _ _ _ _ piles.

Two: true or false: (tick after the question you think is correct and tick after the question you think is wrong)

(1)20 is divisible by 4 ().

(2) 1 is both odd and even ().

(3) 1 is neither a prime number nor a composite number ().

(4) Incomplete composite numbers are even numbers, and incomplete prime numbers are odd numbers ().

(5)52= 13×4, 13 and 4 are all factors of 52. ( )。

(6) Because 52 =13× 4; So we say that the common multiple of 13 and 4 is only 52. ( )

(7) Two numbers of coprime must both be prime numbers ().

(8) Two prime numbers must be coprime ().

(9) The product of two numbers must be the common multiple of these two numbers ().

(10) The least common multiple of two numbers must be the multiple of the greatest common factor of these two numbers ().

Three: multiple choice questions:

(1)48 has () all factors * * *.

9 (B)8 (C) 10 (D) 12。

(2) In 14 = 2× 7, both 2 and 7 are () of 14.

(a) prime number (b) coprime number (c) prime factor (d) common factor

(3)a and B are prime numbers, and their least common multiple is ().

(A)a (B)b (C) 1 (D)ab

(4) When a rectangular house is decorated with square floor tiles, the length and width of the house should be multiple of the side length of the square, and the side length of the square floor tile should be the length and width of the rectangle ().

(a) Common factor (b) Maximum common factor (c) Common multiple (d) Minimum common multiple

(5) There are two buses at a bus stop. Bus A leaves every minute, and bus B leaves every minute. After these two buses leave at the same time, the next bus leaves at the same time is A and B ().

(a) Common factor (b) Maximum common factor (c) Common multiple (d) Minimum common multiple

Four: Find the greatest common factor and the least common multiple of the following groups by short division. (Each small question 10, 20 points)

(1)42 and 63 (2)8 and 20

Five: Students in a school do problems and are divided into 10 group, 14 group and 18 group. Just after that, it is known that the number of students in this school exceeds 1000. How many students are there at least in this school?

Six: First decompose 42 and 30 into prime factors, and then answer the following questions:

( 1)42= 30=

(2) All common prime factors of 42 and 30 are

(3) The only prime factor of 42 and 30 is

(4) The greatest common factor of 42 and 30 is

(5) The least common multiple of 42 and 30 is

(6) Through the above answers, you can sum up that

Seven: Let A be a positive even number greater than 3, then the even number below it can be expressed as A-2, and the even number above it can be expressed as A+2. Because A+(A-2)+(A+2) = 3a, we can say that the sum of three consecutive even numbers can be divisible by 3. Try the above method to explain that "the sum of three consecutive positive integers can be divisible by 3".

No.8 bus: No.3 bus runs every 6 minutes, and No.5 bus runs every 8 minutes. After they left a station at the same time at 6: 00 in the morning, how many minutes did it take them to leave at the same time for the second time? When did we meet for the third time?