Xét dấu của tam thức sau \(f(x) = - 4{x^2} + 12x - 9\).

2/22

\( - 4{x^2} + 12x - 9 < 0,\,\,\forall x \in \mathbb{R}\backslash \left\{ { - \frac{3}{2}} \right\}\).

\( - 4{x^2} + 12x - 9 > 0\,,\,\forall x \in \mathbb{R}\backslash \left\{ {\frac{3}{2}} \right\}\).

\( - 4{x^2} + 12x - 9 < 0\,,\,\forall x \in \mathbb{R}\backslash \left\{ {\frac{3}{2}} \right\}\).

\( - 4{x^2} + 12x - 9 > 0,\,\,\forall x \in \mathbb{R}\backslash \left\{ { - \frac{3}{2}} \right\}\).

Giải thích

Ta có \(\Delta ' = 0,\,\,a < 0\) suy ra \( - 4{x^2} + 12x - 9 < 0,\,\,\forall x \in \mathbb{R}\backslash \left\{ {\frac{3}{2}} \right\}\).