How can i define if a number is prime or composite?

Answer 1

For relatively small numbers, test the prime numbers less than the square root of the number. If the number is composite, one of these prime numbers will divide it.

For very large numbers that do not have an obvious factor, there is no easy way. That is why prime numbers are used in cryptography.

Answer 2

if the number has two factors, it is prime. if it has 3+ factors, it is composite. for example, 2, 3, 5, 7, 11, 13, 17, 19, 23 are all prime. 4,6,8,9,10, 12, 14, 15, 16, 18, 20, 21, 22 are all composite.

Answer 3

-If it is even, it is composite : 152 is composite

-If you add its digits, and they add to 3 6 or 9 it is composite : 9721 is composite

-If it ends in 5, it is composite : 1355 is composite

-If you double the last digit, and subtract from the rest of the digits, and your answer is in the 7x table, it is composite : 371 = 37 - 2 = 35 -- in the 7x table -- composite

- If it is a digit doubled or tripled, it is composite : 99 is composite as is 777

- if the sum of all the even digits subtracted from the sum of all the odd digits = 0 or 11 it is composite : 135377 = (1 + 5 + 7) - (3 + 3 + 7) = 13 - 13 = 0 -- composite

Other tests are possible, but these catch many composites that look prime.

Answer 4

An integer p > 1 is prime if it has no integer factors greater than 1 and less than p. If it does have such a factor, it is composite.

Answer 5

If the number has any amount of factors other than 2, then it is composite.

A prime number has exactly two factors - itself and 1.