có :UCLN(a,b)=30 va BCNN(a,b)=60
=>a.b=UCLN(a,b).BCNN(a,b)=30.60=1800
ma UCLN(a,b)=30
=>a chia hết cho 30=>a=n.30
b chia hết cho 30=>b=m.30
=>a.b=n.30.m.30=1800
=900.n.m
=>n.m=1800:900=2
=>n.m=2
=> cac cap (a;b)=(30;60) ; (60;30)
vay (a;b)=(30;60) ; (60;30)