Cho tam thức f( x ) = a{x^2} + bx + c\

4/11

Cho tam thức \(f\left( x \right) = a{x^2} + bx + c\left( {a \ne 0} \right),\Delta = {b^2} - 4ac\). Khi đó \(f\left( x \right) \le 0,\forall x \in \mathbb{R}\) khi

\(\left\{ \begin{array}{l}a < 0\\\Delta \le 0\end{array} \right.\).

\(\left\{ \begin{array}{l}a \le 0\\\Delta < 0\end{array} \right.\).

\(\left\{ \begin{array}{l}a > 0\\\Delta \le 0\end{array} \right.\).

\(\left\{ \begin{array}{l}a < 0\\\Delta \ge 0\end{array} \right.\).

Giải thích

\(f\left( x \right) \le 0,\forall x \in \mathbb{R}\)\( \Leftrightarrow \left\{ \begin{array}{l}a < 0\\\Delta \le 0\end{array} \right.\). Chọn A.