# Arithmetic/Types of Numbers/Real Number

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The collection of all rational numbers and irrational numbers make up what is called the Real Number system.

A number that can be expressed as a ratio of integer is a rational number.

A number that is not a rational number is an irrational number.

The following are some common irrational numbers:

- π (which can be approximated to 3.14159265 and represents the length of the circumference of a circle divided by its diameter)
- √2 (which is the square root of 2 and is approximately 1.4142)
- e (which is the base of the natural logarithms and is approximately 2.71828)

- and so on.