Bài 1. \({A_1} = \left\{ {{\rm{ }}a;b;c{\rm{ }}} \right\};{A_2} = \left\{ {a;b;d{\rm{ }}} \right\};\)\(\,{\rm{ }}{A_3} = \left\{ {a;c;d} \right\}{\rm{ }};{A_4} = \left\{ {b;c;d} \right\}.\)
Bài 2. \(B = \{5;15;25;35;45;55;65;75;85;95\}\).
Bài 3. \(C = \{0;1;2\}\).