## Thinking Mathematically (6th Edition)

$\pi$ is a real number but is not a rational number.
Rational numbers are the numbers that can be expressed as a quotient of two integers. Thus, a number that cannot be expressed as a quotient of two integers is not a rational number. Such a number is called an irrational number. The number $\pi$ is not a rational number as it cannot be expressed as a quotient of two integers. $\pi = 3.14159265359...$ Thus, one example of a real number that is not a rational number is $\pi$.