Tìm x biết ​\(x \in BC\left( {6;12} \right), 0 jwplayer.key="...

* Hướng dẫn giải

 \(\begin{array}{l} 6 = 2.3\\ 12 = {2^2}.3\\ \Rightarrow BCNN\left( {6;12} \right) = {2^2}.3 = 12\\ \Rightarrow BC\left( {6;12} \right) = B\left( {12} \right) = \left\{ {0;12;24;36;48;60;72;84;96;108;120;132;144;...} \right\}\\ \text{mà}\,0 < x < 20 \Rightarrow x = 12 \end{array} \)