# Game – Prime Number Recognition

## You will need:

2 dice and a pen and paper.

## Take in turns to:

• Throw the dice
• From the dice, construct 2 or perhaps 3 numbers. For example, if you throw a two and a three, you can make 5, 23 and 32 (3+2=5, two followed by three is 23 and three followed by two is 32)
• Score one point for each prime number you have made (so this example scores one for the 5 and one for the 23, scoring two points in total).
• If you need to use a calculator, then a Casio fx-83GT PLUS can tell you whether a number is prime. This is *not* cheating – students will soon start to recognise the primes they need, rather than having to check using the calculator!

## The Winner Is:

The person who has the most points.

## Things to discuss:

• Why are two even numbers always such bad news? (even+even=even and the only even prime number is two)
• Is it possible to score three points with one throw?
• If there is a six in your throw, what happens?

This sample space diagram may help:

 Sample Space Diagram for 2 Dice 1 2 3 4 5 6 1 (1,1) (2,1) (3,1) (4,1) (5,1) (6,1) 2 (1,2) (2,2) (3,2) (4,2) (5,2) (6,2) 3 (1,3) (2,3) (3,3) (4,3) (5,3) (6,3) 4 (1,4) (2,4) (3,4) (4,4) (5,4) (6,4) 5 (1,5) (2,5) (3,5) (4,5) (5,5) (6,5) 6 (1,6) (2,6) (3,6) (4,6) (5,6) (6,6)

# How to Factorise a Number (Or Check that is Prime) using a CASIO fx-83GT PLUS calculator

On the CASIO fx-83gt PLUS factorising is done like this:

• Entering the number,
• press equals,
• SHIFT and ., ,,, (this has “FACT” written above it in yellow).
• The Prime Factor Form is displayed as the answer.
• If the number is Prime, then the number itself is displayed.

This is a natural way to introduce what indices mean, because the CASIO gives the answers in index form eg 34 rather than 3x3x3x3