Rounding Since some decimals go on forever, we are often asked to ‘round’ them to a certain number of decimal places.
Rules for rounding
If, for example, we are asked to round a decimal correct to two decimal places, we need to look at the number in the third decimal place.
If the number in the third decimal place is less than 5, leave the fi rst two decimal numbers unchanged.
If the number in the third decimal place is 5 or more, increase the second decimal number by one.
Worked example 1 (i) Round 3·27 to one decimal place.
Solution (i)
‘Zero to four, just ignore; Five to nine, one to climb’
(ii) Round 5·1284 to three decimal places.
(ii)
Signifi cant fi gures
Another method of giving an approximate answer is to round using signifi cant fi gures.
For example, if we want to know how many people attended a football match, a fi gure of 32 000 would be acceptable even if the exact number was 31 964.
To round to a number of signifi cant fi gures, count the numbers from left to right, and then round off from there using the rules of rounding.
When there are leading zeros (such as 0·006), don’t count them because they are only there to show how small the number is.
For example: ● ●
●
2·339 rounded to 3 signifi cant fi gures is 2·34 125·7 rounded to 1 signifi cant fi gure is 100 0·0168 rounded to 2 signifi cant fi gures is 0·017
Section A Introducing concepts and building skills