2.55. Trang 56

a) Ta có: 21 = 3.7;  98 = 2.$7^{2}$

=> ƯCLN(21, 98) = 7 ; BCNN(21, 98) = 2.3.$7^{2}$ = 294

b) Ta có: 36 = $2^{2}$.$3^{2}$, 54 = 2.$3^{3}$

ƯCLN(36, 54) = 2.$3^{2}$ = 18; BCNN(36, 54) = $2^{2}$.$3^{3}$ = 108.