1 Prime & Composite Numbers Stage 4 - Year 7 Press Ctrl-A ©2009 – Not to be sold/Free to use

2 A prime number has two (and only two) factors: itself and 1. A composite number has more than 2 factors. Example 1: 7 Factors are only 7 and 1. 7 is a Prime Number. Example 2: 8 Factors are 8, 4, 2 and 1. 8 is a Composite Number. (NB 1 is neither Prime or Composite) Prime & Composite Numbers 42

3 To test whether a number is prime or composite: Test all primes less than or equal to the square root of the number. If the number is not divisible by any of the above then the number is prime. 42 Prime & Composite Numbers

4 Prime Number Trick: Using prime numbers, you can amaze your friends with a prime prediction... Ask your friends to pick any prime number bigger than 5, but they must not tell you what it is. Add 17 Without knowing which prime number your friends picked, you can still tell them: There will be a remainder of 6. (Or if they do the sum on a calculator, the answer will end with ".5") Square it. Divide by Prime & Composite Numbers

5 If you square ANY prime number bigger then 3, then subtract 1, the answer always divides by 24! E.g. 112 = 121 then = 120 and yes 120 does divide by 24. The "24" Mystery! 42 Prime & Composite Numbers

6 Aliens know primes !? 42 Prime & Composite Numbers

7 Speed Limits The speed limit is 31 in downtown Trenton Tennessee, USA Prime Curios! Sieve of Eratosthenes Can be used to find Primes 42 Prime & Composite Numbers