D=\(\left(\sqrt{2}x\right)^2-2\sqrt{2}x.\frac{1}{\sqrt{2}}+\frac{1}{2}-\frac{1}{2}-5=\left(\sqrt{2}x-\frac{1}{\sqrt{2}}\right)^2-\frac{11}{2}\)
vi (\(\left(\sqrt{2}x-\frac{1}{\sqrt{2}}\right)^2\)>0
nen \(\left(\sqrt{2}x-\frac{1}{\sqrt{2}}\right)^2\)-\(-\frac{11}{2}\)> \(\frac{-11}{2}\)
D dat GTNN la -11/2 khi x=1/2