Cho \(f\left( x \right) = a...

Câu hỏi: Cho \(f\left( x \right) = a{x^2} + bx + c\,\left( {a \ne 0} \right)\). Điều kiện để \(f\left( x \right) \le 0\,,\forall x \in R\) là

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

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

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

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