Intermediate Algebra for College Students (7th Edition)

A rational number is a number that can be expressed as a quotient of two integers. Note that $9 = \frac{18}{2}$. 18 and 2 are integers. Thus, $9$ is a rational number. $\in$ means that the number is an element of the given set. Since $9$ is a rational number, it is an element of the set of rational numbers. Therefore, the given statement is true.