# Maths Topic 16 - Special Numbers

Special Numbers are commonly tested in 11+ exams and UK Selective School.

Primes, even, odd, square numbers, square roots, cube numbers, triangular numbers, Roman numerals, negative numbers and indices.

## Examples

### Example Question 1:

Which of the following answer options contains all prime numbers?

(a) 2, 4, 6, 8, 10,  (b) 1, 3, 5, 7, 11,  (c) 11, 13, 15, 17, 19,  (d) 2, 3, 7, 13, 17,  (e) 9, 16, 25, 36, 49

Explanation: Prime numbers can only be divided by themselves and 1. In answer option (d) all of the numbers have these 2 factors.

### Example Question 2:

(a) What is 8² ?

(b) What is √81?

(c) What is the third triangular number?

(d) What is 3³ ?

Answers: (a) 64,  (b) 9,  (c) 6,  (d) 27

Explanations:

(a) 8² is 8 × 8 = 64

(b) √81 is what number was multiplied by itself to make 81 (the square root) = 9

(c) Triangular numbers add 1 more on to each number, starting with the number 1: 1 + 2 = 3, 1  + 2 + 3 = 6

(d) 3³ = 3 × 3 × 3 = 27