Đáp án:
$\begin{array}{l}
a)\\
+ )220 = {2^2}.5.11\\
240 = {2^4}.3.5\\
300 = {2^2}{.3.5^2}\\
\Rightarrow UCLN\left( {220;240;300} \right) = {2^2}.5 = 20\\
\Rightarrow BCNN\left( {220;240;300} \right) = {2^4}{.3.5^2}.11 = 13200\\
+ )40 = {2^3}.5;75 = {3.5^2};105 = 3.5.7\\
\Rightarrow UCLN\left( {40;75;105} \right) = 5\\
\Rightarrow BCNN\left( {40;75;105} \right) = {2^3}{.3.5^2}.7 = 4200\\
+ )18 = {2.3^2};36 = {2^2}{.3^2};72 = {2^3}{.3^2}\\
\Rightarrow UCLN\left( {18;36;72} \right) = {2.3^2} = 18\\
\Rightarrow BCNN\left( {18;36;72} \right) = {2^3}{.3^2} = 72\\
b)A = 5.6.7 + 8.9 = 2.\left( {5.3.7 + 4.9} \right) \vdots 2\\
B = 51.7.9.11 - 2.3.7.17 \vdots 7\\
C = 5.7.11 + 13.17.19 \vdots 2\\
D = 4253 + 1422 = 5675 \vdots 5
\end{array}$
