A decimal number is a number that is written with an integer part and a finite decimal part.
Decimal numbers of this form are part of the rational numbers.
The two parts of a decimal number are separated by a decimal point to the right of the ones. The part of the number to the left of the decimal point is called the integer part, and the part of the number to the right of the decimal point is called the decimal (or fraction) part.
In the number |15{.}2,| the integer is |\color{red}{15}| and the finite decimal part is |\color{blue}{2}.|
The following are more examples of decimal numbers:
|\color{red}{16}{.}\color{blue}{231}|
|\color{red}{5}{.}\color{blue}{6}|
|\color{red}{98}{.}\color{blue}{123456}|
|\color{red}{0}{.}\color{blue}{25}|
When the decimal part of a number is repeated, that number is sometimes defined as a periodic number.
We call the repeating sequence a period.
Periodic numbers are also part of rational numbers.
To indicate a number’s decimal part is periodic, we draw a line above the repeating sequence (i.e., the period).
|2{,}66666666...=2{.}\overline{6}|
The period is |6.|
|65{.}987987987...=65{,}\overline{987}|
The period is |987.|