Ta có :
\(G=-5x^2+7x-3\)
\(\Rightarrow G=-\left(5x^2+7x+3\right)\)
\(\Rightarrow G=-\left[x^2+2x.\frac{7}{2}+\left(\frac{7}{2}\right)^2-\left(\frac{7}{2}\right)^2+4x^2\right]\)
\(\Rightarrow G=-\left[\left(x+\frac{7}{2}\right)^2+\frac{49}{4}-3+4x^2\right]\)
\(\Rightarrow G=-\left[\left(x+\frac{7}{2}\right)^2+\frac{37}{4}+4x^2\right]\)\(\Rightarrow G=-\left(x+\frac{7}{2}\right)^2-\frac{37}{4}-4x^2\)
\(\Rightarrow G< 0\forall x\)
\(H=-4x^2-6x-4\)
\(\Rightarrow H=-\left(4x^2+6x+4\right)\)
\(\Rightarrow H=-\left[\left(2x\right)^2+2.2x.\frac{3}{2}+\left(\frac{3}{2}\right)^2+\frac{7}{4}\right]\)
\(\Rightarrow H=-\left[\left(2x+\frac{3}{2}\right)^2+\frac{7}{4}\right]\)
\(\Rightarrow H=-\left(2x+\frac{3}{2}\right)^2-\frac{7}{4}< 0\forall x\)
