Rationalise the denominator of the following fraction: 1/(√2 + 1)
We start with 1/(√2 + 1)
Normally with rationalising surd denominators we multiply the top and bottom of the fraction by the denominator. But this time we have a surd ADDED by a rational number.
In this case we multiply the top and bottom by the denominator with the connecting + or - sign REVERSED ie by:(√2 - 1)
So we get
(√2 - 1) / (√2 + 1)(√2 - 1)
The bottom is multiplied out like a quadratic... a special type of quadratic [(a - b)(a + b)]. A handy but not vital rule to remember is:
(a - b)(a + b) = a2 - b2
So back to our fraction, we get
(√2 - 1) / (2 - √2 + √2 - 1)
= (√2 - 1) / (2 - 1)
= (√2 - 1) / 1
= √2 - 1 --> our final answer!
If you would like more examples, as usual BBC Bitesize is good at walking through the solutions to a number of types of questions on surds: http://www.bbc.co.uk/education/guides/z7fbkqt/revision/2
