Prime Numbers
Prime numbers
A prime number is a number with exactly two factors. A prime number is only divisible by 1 and itself. Another way to think of prime numbers is that they are only ever found as answers in their own times tables.
11 is a prime number because the only factors of 11 are 1 and 11 (\(1 \times 11 = 11\)). No other whole numbers can multiply together to make 11.
15 is not a prime number because the factors of 15 are 1, 3, 5 and 15 (\(1 \times 15 = 15, 3 \times 5 = 15\)).
15 has more than 2 factors, so it is not a prime number.
1 is not a prime number as it only has one factor - itself.
2 is the only even prime number. Every even number has 2 as a factor, and so will not be a prime number.
There are an infinite number of prime numbers. The prime numbers under 30 are: 2, 3, 5, 7, 11, 13, 17, 19, 23, and 29.
Example
Identify a prime number from this list: 42, 43, 44, 45
42 and 44 are even, and so cannot be a prime number as they both have 2 as a factor.
45 = \(5 \times 9\) and so is not a prime number.
43 is a prime number as the only factors of 43 are 1 and 43.
Square Numbers
A square number is the answer when an Integers are whole numbers. is multiplied by itself. It is called a square number because it gives the area of a square whose side length is an integer.
The first square number is 1 because \(1 \times 1 = 1\).
The second square number is 4 because \(2 \times 2 = 4\).
The third square number is 9 because \(3 \times 3 = 9\), and so on.
The first fifteen square numbers are: 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196 and 225.
Example
What is the twentieth square number?
20 squared = \(20 \times 20 = 400\)
So 400 is the twentieth square number.
Cube numbers
A cube number is the answer when an integer is multiplied by itself, then multiplied by itself again. It is called a cube number because it gives the volume of a cube whose side length is an integer.
The first cube number is 1 because \(1 \times 1 \times 1 = 1\).
The second cube number is 8 because \(2 \times 2 \times 2 = 8\).
The third cube number is 27 because \(3 \times 3 \times 3 = 27\), and so on.
The first five cube numbers are: 1, 8, 27, 64 and 125.
Example
What is the tenth cube number?
10 cubed = \(10 \times 10 \times 10 = 1000\)
So 1000 is the tenth cube number.