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The set of prime numbers less than 20 is {2.3.5.7.11.13.17.19} prime numbers are prime numbers The set of prime numbers less than 20 in grade one is {2.3.5.7.11.13.17.19}. Prime numbers are prime numbers, and prime numbers cannot be divisible. Why is 2 prime?
Because 2 also satisfies the prime condition: only 1 and itself are its factors
2. 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 odd numbers: even numbers: prime numbers:
The total number is:
Odd: 3 57 9 11 13 15 17 19 even: 2 46 8 10 12 14 16 18 prime; 2 57 11 13 17 19 combined; 4 68 9 10 12 14 15 16 18
At 1, 3, 5 How many times does the number 5 appear in 697 and 699?
Appear in 100 places: 501-599, 50 times
It appears in the ten digits 51-59151-159251-259351-359451-459551-559651-659
A total of 5 * 7 = 35 times
It appears in position 05-695
70 times in all
All in all, 155 times
RELATED INFORMATIONS
- 1. The numbers in the positive odd number sequence 1,3,5,7... Are arranged into the following triangle number table according to the principle of "top small, bottom large, left small, right large" one 3 5 7 9 11 13 15 17 19 ……… Please judge the number of 2009 in the triangle number table The detailed process is satisfactory
- 2. Among the odd numbers 1.3.5.697 and 699, how many times does the number 5 appear?
- 3. Finding the sum of the first n terms of sequence 3,7,11,15.2n-1
- 4. The observed values (1), (3,5), (7,9,11) (13,15,17,19) were in which group Please write down the calculation or reasoning process
- 5. 1、153°39'+25°40'38" 2、90°-37°24‘38” 3、25°53’8“×5 4、15°20‘36
- 6. Simple and convenient operation
- 7. Simple method of 35 × 35 + 65 × 35
- 8. 123×123＋123
- 9. Why is the square of 3:1 equal to 9:3:3:1
- 10. According to the Pythagorean theorem, the square of a plus the square of B equals the square of C. how to calculate the square of 35 plus 12 equals the square of 50
- 11. How to calculate 10 * 7 / 15 =?
- 12. The numbers 1, 2, 3,... Are written on the blackboard , 2009, arbitrarily erase two numbers a and B, and then rewrite them as the difference between the two numbers, so continue until If there is only one number left, is the last number left odd or even?
- 13. There are 1, 3 and 5 on the blackboard. There are 1000 numbers in total. Erase two numbers at any time, and then write the sum of the two numbers After how many erasures, there are only two numbers left on the blackboard?
- 14. Additional questions: write 2000 numbers on the blackboard: 1, 2, 3 , 2000, it is allowed to erase two numbers a and B (a ≥ b) each time and write the three numbers A-B, AB and ab (that is, AB writes twice). After 8000 times of doing so, 10000 numbers are obtained. Q: can these 10000 numbers all be less than 500?
- 15. Mr. Wang wrote three integers 2,4,8 on the blackboard. Then he erased a number and added a number, which is more than the sum of the two numbers on the blackboard. For example, he erased 4 and added a number 2 + 8 + 1 = 11. At this time, the number on the blackboard is 2,8,11?
- 16. Write 1, 2,... On the blackboard As long as there are two or more numbers on the blackboard, erase any two of them a and B, and write A-B (where a ≥ b), and ask whether the last one on the blackboard is odd or even
- 17. The blackboard says 0.01, 0.02, 0.03 Then, 1, these 100 numbers, each time arbitrarily erase two of them a, B, and write 2ab-a-b + 1, and ask what is the number left on the blackboard? Why?
- 18. There are 2003 numbers on the blackboard. Erase two numbers at any time and write another one______ After that, there is only one number left on the blackboard
- 19. 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = simple algorithm
- 20. A simple calculation method of 2.8 × 1.6 + 4.25.4 × 2.3-7.64 To say the reason to write complete Oh!