Giải thích các bước giải:
Gọi $(a,b)=d\to a=md, b=nd (m,n)=1\to [a,b]=mnd$
Giả sử $a\ge b\to m> n$
$\to mnd-d=18$
$\to d(mn-1)=18$
$\to d,mn-1 \in U(18)\to(d,mn-)\in\{(18,1), (9,2),(6,3), (3,6), (2,9),(1,18)\}$
$\to(d,mn)\in\{(18,2), (9,3),(6,4), (3,7), (2,10),(1,19)\}$
$\to(d,m,n)\in\{(18,2,1), (9,3,1),(6,4,1), (3,7,1), (2,10,1),(2,5,2),(1,19,1)\}$
$\to (a,b)\in\{(36,18), (24,6), (21,3), (20,2), (20,4), (19,1)\}$
