Rational and Irrational Numbers



Rational and Irrational Numbers

A rational number can be written as fraction, so not surprisingly an irrational number can not be written as a fraction.

1.5 is rational because it can be written as [pic] as is 6 [pic]and 0.23 [pic]

All integers are rational.

An irrational number will be a decimal which goes on forever (non-terminating) without ever repeating.

The main examples of irrational numbers are surds (see later) and multiples of and π.

We can express irrational numbers in terms of π and surds

e.g. Write down an irrational number between 7.2 and 7.3

π = 3.141……… so π + 4.1 will be irrational and between 7.2 and 7.3

[pic] will be irrational and lie between 7.2 and 7.3

Writing a Non-Terminating Recurring Decimal as a Fraction

If a decimal goes on forever (non-terminating) but repeats, then it is rational i.e. it can be written as a fraction. The obvious example is 0.333333333… which is [pic]

Some numbers are not quite so obvious but can still be written as fractions

e.g. Write 0.[pic] as a fraction

Let x = 0.444444….. so 10x = 4.4444444…..

10x – x = 4 = 9x

If 9x = 4 then x = [pic] but as x = 0.[pic] 0.[pic]= [pic]

e.g. Write 3.[pic] as a fraction

We know that 0.[pic] = [pic] so 3.[pic] must equal [pic]

e.g. Write [pic] as a fraction

[pic]

Let x=[pic] so 100x = 23.232323…….

100x – x = 23.23232323…. – 0.23232323….. = 23 = 99x

If 99x = 23 then x = [pic] but x = 0.23232323…..

So [pic] = [pic]

After a while you can just recognise the answers if the recurring part of the decimal is next to the decimal point

[pic]

However you must remember the ‘long-winded’ method as you may be asked to prove that, for example, [pic] = [pic]

If the recurring part is not next to the decimal point then we have to split the recurring and non-recurring parts and use a fancy bit of division/multiplication to put the recurring part next to a decimal point. This is probably going to appear on the Higher paper.

e.g. Write 0.2[pic] as a fraction

0.2[pic] = 0.2 + 0.0[pic] Split into recurring and non-recurring parts

= 0.2 + 0.[pic] ÷ 10 Move recurring part next to decimal point

[pic]

e.g. Write 0.41[pic] as a fraction

0.41[pic] [pic]

You can check your answer by converting the fraction to a decimal by dividing and seeing if you get the starting value e.g. [pic]

Surds

Surds are the square root of a prime number (or a multiple thereof).

They are always irrational.

It is more accurate to write your answer as a square root rather than as a decimal if you are dealing with surds – particularly useful in situations using Pythagoras’ Theorem

There are two laws of surds which are often used to simplify expressions:

Law 1 [pic] (and vice versa)

Law 2 [pic]

e.g. Simplify [pic]

[pic]

The trick is to look for the largest square number (the square root of which will always be an integer) which will divide into your expression.

e.g. Simplify [pic]

[pic]

Rationalising a Fraction

If we rationalise a fraction containing a surd, we write it so that there are no surds in the denominator (bottom). There will probably be some surds left in the numerator.

Rationalising does not mean ‘get rid of all the surds’.

The secret to rationalising is to use equivalent fractions and multiply by one which will not change the value of our expression

(we just write one in a very clever way)

e.g. Rationalise [pic]

[pic]

e.g. Rationalise [pic]

[pic]

Higher Level

If you have a more complicated denominator then you need to be aware of the difference of 2 squares i.e. a2 – b2 = (a – b)(a + b)

e.g. If you have to rationalise [pic] there is no point multiplying by [pic]

as when you multiply out the denominators you will get:

[pic]

This means we will still have a surd in our denominator so it has not been rationalised

The difference of 2 squares would suggest that we multiply by [pic]

[pic]

There are no surds in the denominator so it has been rationalised.

You may prefer to write it without expanding the numerator i.e.

[pic]

Past Paper Questions

[pic]

[pic]

[pic]

[pic]

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Write as fractions

Perform the division

Find common denominator and re-write using equivalent fractions

Add up

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