# Number

Revision notes on numbers

- Created by: Umar Nurgat
- Created on: 20-04-09 16:49

First 130 words of the document:

NUMBERS

Prime Numbers

Prime numbers don't divide by anything except 1 and themselves.

1 is not a prime number

The first four prime numbers are 2, 3, 5, and 7

ALL prime numbers end in 1, 3, 7 or 9

NOT ALL numbers ending in 1, 3, 7 or 9 are prime numbers.

Finding Prime Numbers

For a number to be prime:

E.g. "Decide whether or not 233 is a prime number."

1) Does it end in 1, 3, 7 or 9? Yes

2) Find its square root: 233 = 15.264

3) List all primes which are less than this square root: 2, 3, 5, 7, 11 and 13

4) Divide all of these primes into the number under test:

233/3 = 77.6667 233/7 = 33.2857 (obviously won't divide by 5 and 2)

233/11= 21.181818 233/13= 17.923077

5) Since none of these divide cleanly into 233 then it is a prime number.

1

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