If $n=\frac{a}{b}$ , with $n, a$ and $b$ integers $( b ≠ 0)$, and with $a$ and $b$ coprimes, does $b=1$?
Show that if $fd'=f'd $ and the pairs $f, d $ and $f',d' $ are coprime, then $f=f' $ and $d=d' $.