Key facts to remember:
- Prime numbers do not divide by anything except themselves and 1
- All prime numbers end in 1, 3, 7, or 9
How to find out if a chosen number is a prime number:
- Does the number end in a 1, 3, 7 or 9?
- If so, can it be divided by any of the prime numbers that are smaller than its square root?
- If not, then it is a prime number
Is 83 a prime number?
It ends in a 3, so it could be
The square root of 83 is 9.11 (to 3 s.f.) Prime numbers smaller than 9.11 are 2, 3, and 7. 83 cannot be divided by 2, 3, or 7 so 83 is a prime number.
Factors of a number are all the numbers that divide into it
Finding all the factors of a given number
Example, find all the factors of 24
1 x 24
2 x 12
3 x 8
4 x 6
5 x –
6 x 4
List the factors in pairs, starting with 1 x the number itself. Work your way through methodically, trying 1, 2, 3,…
If you come to a number that won’t divide, put a dash by it, like 5 in the example above
Once a number is repeated, like 4 in the example, you have found all the factors
To find a number as a product of its prime factors, use a factor tree.
Start with your number at the top, split it into two factors, split each factor until you get a prime factor
Multiples of a number are the number multiplied by another number.
For example, some multiples of 12 are 24, 36, 48, 60, 72, 84, 96, 108, 120, ……and so on
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