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GCSE Maths > Number - Surds

Surds are numbers left in 'square root form' (or 'cube root form' etc). They are therefore irrational numbers. The reason we leave them as surds is because in decimal form they would go on forever and so this is a very clumsy way of writing them.

Addition and Subtraction of Surds

Adding and subtracting surds are simple- however we need the numbers being square rooted (or cube rooted etc) to be the same.

47 - 27 = 27.
52 + 82 = 132

Note: 52 + 33 cannot be manipulated because the surds are different (one is 2 and one is 3).

Multiplication

5 15 = 75 (= 15 5)
= 25
3
= 53.

(1 + 3) (2 - 8) [The brackets are expanded as usual]
= 2 - 8 + 23 - 24
= 2 - 22 + 23 - 26

Rationalising the Denominator

It is untidy to have a fraction which has a surd denominator. This can be 'tidied up' by multiplying the top and bottom of the fraction by a particular expression. This is known as rationalising the denominator, since surds are irrational numbers and so you are changing the denominator from an irrational to a rational number.

Example

Rationalise the denominator of:
a) 1
2 .

b) 1 + 2
1 - 2

a) Multiply the top and bottom of the fraction by 2. The top will become 2 and the bottom will become 2 (2 times 2 = 2).

b) In situations like this, look at the bottom of the fraction (the denominator) and change the sign (in this case change the plus into minus). Now multiply the top and bottom of the fraction by this.

Therefore:

1 + 2 = (1 + 2)(1 + 2) = 1 + 2 + 2 + 22 = 3 + 32
1 - 2 (1 - 2)(1 + 2)

1 + 2 - 2 - 2

- 1


= -3(1 + 2)

Copyright Matthew Pinkney 2003

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