\(\text{Ta có: }x\ \vdots\ 12,\ x\ \vdots\ 21,\ x\ \vdots\ 28\)
\(\Rightarrow x\in BC(12;21;28)\)
\(BCNN(12;21;28)\)
\(12=2^2 .3\)
\(21=3.7\)
\(28=2^2 .7\)
\(\Rightarrow BCNN(12;21;28)=2^2.3.7=84\)
\(\Rightarrow x\in BC(12;21;28)=\{0;84;168;252;...\}\)
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