When a number is divisible by only one and itself, then it is a prime number. The prime numbers cannot be factorised as they are not divisible. The divisible numbers are called composite numbers. 1 is neither prime nor composite.
Every prime number can be written in the form of 6n + 1 or 6n – 1 (except the multiples of prime numbers, i.e. 2, 3, 5, 7, 11), where n is a natural number.
To know the prime numbers greater than 40, the below formula can be used.
n2 + n + 41, where n = 0, 1, 2, ….., 39
How do we get to know if a number is prime or not?
To calculate we need to find the factors of the number. Take for example 11, 11 is divisible only by 1 and itself. Since it has only two factors, it is a prime number.
Question 1: Find if 53 is a prime number or not?
The factors of 53 are 1 and 53.
So 53 is only divisible by 1 and 53.
So, 53 is a prime number.
Question 2: Check if 64 is a prime number or not?
Solution. The factors of 1, 2, 4, 16, 8.
Hence it is a composite number and not a prime number.