a, Ta có : 10=2.5
12=2^2 . 3
15=3.5
=> BCNN(10,12,15)=2^2.3.5=60
b, Ta có : 8 = 2^3
9 = 3^2
=>BCNN(8,9,11)=2^3 . 3^2 . 11 = 792
c, Ta có : 24=3. 2^3
40=2^3.5
168=2^3 .3.7
=>BCNN(24,40,168)=2^3 . 3.5.7=840