第5章小数

In Decimals, we learned to convert decimals to fractions. Now we will do the reverse—convert fractions to decimals. Remember that the fraction bar indicates division. So $\frac{4}{5}$ can be written $4÷5$ or $5\overline{)4}.$ This means that we can convert a fraction to a decimal by treating it as a division problem.

So far, in all the examples converting fractions to decimals the division resulted in a remainder of zero. This is not always the case. Let’s see what happens when we convert the fraction $\frac{4}{3}$ to a decimal. First, notice that $\frac{4}{3}$ is an improper fraction. Its value is greater than $1.$ The equivalent decimal will also be greater than $1.$

We divide $4$ by $3.$

No matter how many more zeros we write, there will always be a remainder of $1,$ and the threes in the quotient will go on forever. The number $\text{1.333…}$ is called a repeating decimal. Remember that the “…” means that the pattern repeats.

How do you know how many ‘repeats’ to write? Instead of writing $1.333\dots$ we use a shorthand notation by placing a line over the digits that repeat. The repeating decimal $1.333\dots$ is written $1.\bar{3}.$ The line above the $3$ tells you that the $3$ repeats endlessly. So $\text{1.333…}=1.\bar{3}$

For other decimals, two or more digits might repeat. Table 5.5 shows some more examples of repeating decimals.

It is useful to convert between fractions and decimals when we need to add or subtract numbers in different forms. To add a fraction and a decimal, for example, we would need to either convert the fraction to a decimal or the decimal to a fraction.

In Decimals, we compared two decimals and determined which was larger. To compare a decimal to a fraction, we will first convert the fraction to a decimal and then compare the decimals.

When ordering negative numbers, remember that larger numbers are to the right on the number line and any positive number is greater than any negative number.