How_To_Be_Good_At_Math

32 NUMBERS •^ PRIME NUMBERS

Prime numbers

A prime number is a whole number greater than

1 that can’t be divided by another whole number

except for itself and 1.

A prime number has

only two factors

• itself and 1.

Is 4 a prime number?

We can divide 4 by 1 and by itself. Can

we divide 4 exactly by any other number?

Let’s try dividing by 2: 4 ÷ 2 = 2

We can divide 4 by 2, so 4 is not a prime number.

Is 2 a prime number?

We can divide 2 by 1 and also by itself.

But we can’t divide 2 by any other number.

So, we know 2 is a prime number.

Is 7 a prime number?

We can divide 7 by 1 and by itself. Now let’s try

dividing 7 by other numbers. We can’t divide 7

exactly by 2, 3, or 4. We can stop checking once we

get over half of the number we’re looking at – in this

example, once we get to 4. So, 7 is a prime number.

Is 9 a prime number?

We can divide 9 by 1 and by itself.

We can’t divide 9 exactly by 2, but we

can divide it by 3: 9 ÷ 3 = 3

This means 9 is not a prime number.

Finding prime numbers

To find out whether or not a number is prime, we can try to divide it

exactly by other whole numbers. Let’s try this out on a few numbers.

2 ÷ 1 = 2

2 ÷ 2 = 1

4 ÷ 1 = 2

4 ÷ 4 = 1

9 ÷ 1 = 9

9 ÷ 9 = 1

7 ÷ 1 = 7

7 ÷ 7 = 1

YES

2 is a prime

number

YES

7 is a prime

number

NO

4 is not a

prime number

NO

9 is not a

prime number

4 ÷ 2 = 2

9 ÷ 3 = 3

032_033_Prime_Numbers.indd 32 29/02/2016 14:08