"Proof: Suppose \(r\) and \(s\) are rational numbers. If \(r+s\) is rational, then
by definition of rational \(r+s=a / b\) for some integers \(a\) and \(b\) with \(b
\neq 0\). Also since \(r\) and \(s\) are rational, \(r=i / j\) and \(s=m / n\) for some
integers \(i, j, m\), and \(n\) with \(j \neq 0\) and \(n \neq 0\). It follows that
\(r+s=i / j+m / n=\) \(a / b\), which is a quotient of two integers with a nonzero
denominator. Hence it is a rational number. This is what was to be shown.
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