## Definition

The addition operation in the domain of rational numbers $\Q$ is written $+$.

Let:

$a = \dfrac p q, b = \dfrac r s$

where:

$p, q \in \Z$
$r, s \in \Z_{\ne 0}$

Then $a + b$ is defined as:

$\dfrac p q + \dfrac r s = \dfrac {p s + r q} {q s}$

This definition follows from the definition of and proof of existence of the field of quotients of any integral domain, of which the set of integers is an example.