Factors and Multiples. What are prime numbers?. A prime number is:. If a number has only two different factors, 1 and itself, then the number is said to be prime. For example, 7 = 7 x 1 7 is a prime number since it has only two different factors. Clearly, 2 = 1 x 2 3 = 1 x 3
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Factors and Multiples
A prime number is:
If a number has only two different factors, 1 and itself, then the number is said to be prime.
For example, 7 = 7 x 1
7 is a prime number since it has only two different factors.
Clearly, 2 = 1 x 2
3 = 1 x 3
5 = 1 x 5
7 = 1 x 7
11 = 1 x 11
Therefore 2, 3, 5, 7, 11… are all prime numbers.
A composite number is:
A number that has more than two factors is called a composite number.
For example, 14 = 1 x 14 and 2 x 7
So, 14 is a composite number as it has more than two factors.
‘Factors’ are the numbers you multiply to get another number:
2 x 3 = 6
To test if a number is prime or composite by hand, the easiest thing to do is test if its divisible by prime numbers. If none of them divide it, once the numbers you’re dividing by get bigger than the square root of the number you’re testing, you’re done and know it’s prime.
For example, here’s how you would test if 107 is prime:
INTERACTIVE SITE FOR PRIMES AND COMPOSITES
Sieve of Eratosthenes
A prime number is a whole number that has exactly two factors, 1 and itself.
We can use the Sieve of Eratosthenes to find out whether a number is prime or composite.