Non-Zero Rational Numbers Closed under Multiplication

Theorem
The set of non-zero rational numbers is closed under multiplication.

Proof
Recall that Rational Numbers form Field under the operations of addition and multiplication.

By definition of a field, the algebraic structure $\struct {\Q_{\ne 0}, \times}$ is a group.

Thus, by definition, $\times$ is closed in $\struct {\Q_{\ne 0}, \times}$.