a)ƯCLN=6;BCLN=36

b)ƯCLN =12;BCLN=72

A ) 12 = 2^2*3 
18 = 2*3^2
UCLN(12 , 18) = 6

 Ta có :

12 = 2^2 x 3

18 = 2 x 3^2

=) BCNN ( 12; 18 ) = 2^2 x 3^2 = 36.

=) BC ( 12; 18 ) = B ( 36 ) = {0;36;72;108;...}
b) 

Ta có: 24 = 3. 23

36 = 32 . 22

60 = 3 . 22 . 5

⇒⇒ ƯCLN(24, 36, 60) = 3 . 22 = 12

⇒⇒ BCNN(24, 36, 60) = 32 . 22 . 5 = 18