When writing a final answer to calculations, it's normal to round off to a few decimal places (d.p.) at most. Let's look at how rounding off works: it's not as easy as just dropping d.p. As we've seen, the special number pi is approximately 3·14159265359! Here we've given it to 11 d.p. If we round it off to 10 d.p., it becomes 3·1415926536. This is because the digits 59 are closer to 60 than to 50. Simply dropping the 9 would have given 3·1415926535 which is wrong. So to round off, we first look at the decimal place we intend getting rid of. If it's 5 or more, we round up. If it's 4 or less, we round down. e.g. pi to 4 d.p. is 3·1416 since there is a 9 after the 5.
