2 câu trả lời
$ab \ge 0 \Leftrightarrow \left[ \begin{array}{l} \left\{ \begin{array}{l} a \ge 0\\ b \ge 0 \end{array} \right.\\ \left\{ \begin{array}{l} a \le 0\\ b \le 0 \end{array} \right. \end{array} \right..$
$ab \ge 0 \Leftrightarrow \left[ \begin{array}{l} \left\{ \begin{array}{l} a \ge 0\\ b \ge 0 \end{array} \right.\\ \left\{ \begin{array}{l} a \le 0\\ b \le 0 \end{array} \right. \end{array} \right..$