Với [tex]m\leq \frac{9}{4}[/tex]. Theo viét ta có:
[tex]\left\{\begin{matrix} x_{1}+x_{2}=3 & & \\ x_{1}x_{2}=m & & \end{matrix}\right.[/tex]
Ta có: [tex]m\leq \frac{9}{4}\Rightarrow -5m\geq \frac{-9}{4}\Rightarrow 9-5m\geq 9-\frac{9}{4}=....[/tex]
Dấu ''='' xảy ra khi m=9/4