Ta có: \(x^2+2x+3\ge0\forall x\in R\)
Để f(x) dương thì \(\left[{}\begin{matrix}\left\{{}\begin{matrix}3x^2-x>0\\4-x^2>0\end{matrix}\right.\\\left\{{}\begin{matrix}3x^2-x< 0\\4-x^2< 0\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x< 0;x>\dfrac{1}{3}\\-2< x< 2\end{matrix}\right.\\\left\{{}\begin{matrix}0< x< \dfrac{1}{3}\\x< -2;x>2\end{matrix}\right.\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}-2< x< 0;\dfrac{1}{3}< x< 2\\x=\varnothing\end{matrix}\right.\)
Đáp án C
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