Giải:
Ta có:
$21n+7 \vdots 21n+7$
Lại có:
$18n+3 \vdots 21n+7$
$=>18n+3-(21n+7) \vdots 21n+7$
$=>21.(18n+3)-18.(21n+7) \vdots 21n+7$
$=>-63 \vdots 21n+7$
$=>21n+7 ∈U(-63)=${$-63;-21;-9;-7;-3;-1;1;3;7;9;21;63$}
$21n+7=-63<=>n=\frac{-10}{3}(l)$
$21n+7=-21<=>n=\frac{-4}{3}(l)$
$21n+7=-9<=>n=\frac{-16}{21}(l)$
$21n+7=-7<=>n=\frac{-2}{3}(l)$
$21n+7=-3<=>n=\frac{-10}{21}(l)$
$21n+7=-1<=>n=\frac{-8}{21}(l)$
$21n+7=1<=>n=\frac{-6}{21}(l)$
$21n+7=3<=>n=\frac{-4}{21}(l)$
$21n+7=7<=>n=0(n)$
$21n+7=9<=>n=\frac{2}{21}(l)$
$21n+7=21<=>n=\frac{2}{3}(l)$
$21n+7=63<=>n=\frac{8}{3}(l)$
Vậy $n=0$ thì phân số nguyên.
