I am very happy today. I learned a new knowledge "prime number and composite number" in self-study. What is a prime number and what is a composite number? A prime number is the sum of only 1 and its own two factors. This number is called a prime number, such as 7 1, which has only two factors, 7 1 and 1, so 7 1 is a prime number; A composite number is a number with other factors besides 1 and itself. This number is called a composite number, for example, 10. It has four factors, 1, 10,2,5, so10 is a composite number.

Then, under the guidance of my teacher, I learned to find prime numbers in a numerical table: first, cross out 1, then cross out all multiples of 2 except 2, then cross out all multiples of 3, 5 and 7 except 3, 5 and 7, and finally all the prime numbers in this table are left. That's how to find them.

In self-study, I still have two questions I don't understand. Besides looking for prime numbers in books, is there a faster and more accurate way? Why is 1 not a prime number or a composite number?

It turns out that finding prime numbers and finding composite numbers is such an interesting and complicated knowledge.