code class$=$"asciimath"$>$Definition $3\mathrm{.}4\mathrm{.}4.$ Let $($a$)/($b$),($c$)/($d$)$inQ$.$ We define $($a$)/($b$)+($c$)/($d$)=($ad$+$bc$)/($bd$)$ and $($a$)/($b$)*($c$)/($d$)=($ac$)/($bd$).$