Ta có : 120 = 23 * 3 * 5
40 = 23 * 5
=> BCNN(120;40) = 23 * 5 * 3 = 8 *5 * 3 = 120
Vậy BCNN(120;40) =120
Ta có :120=23.3.5 ; 40=23.5
=>BCNN(120;40)=23.5=40
Ta có : 120 = 23 . 3.5 : 40 = 23 . 5
=> BCNN ( 120 : 40 ) 23.5 = 40
120=2^3*3*5
40=2^3*5
suy ra BCNN(120,40)=2^3*3*5=120