a) 9x² - 6x + 2 = (3x)² - 2.3x.1 + 1² + 1 = (3x - 1)² + 1 mà\(\left(3x+1\right)^2\ge0\Rightarrow\left(3x+1\right)^2+1\ge1>0\)

b) x² + x + 1 = x² + 2.x.\(\frac{1}{2}+\left(\frac{1}{2}\right)^2+\frac{3}{4}=\left(x+\frac{1}{2}\right)^2+\frac{3}{4}\)mà\(\left(x+\frac{1}{2}\right)^2\ge0\Rightarrow\left(x+\frac{1}{2}\right)^2+\frac{3}{4}\ge\frac{3}{4}>0\)

c) 2x² + 2x + 1 =\(\left(\sqrt{2}x\right)^2+2\sqrt{2}x.\frac{1}{\sqrt{2}}+\left(\frac{1}{\sqrt{2}}\right)^2+\frac{1}{2}=\left(\sqrt{2}x+\frac{1}{\sqrt{2}}\right)^2+\frac{1}{2}\ge\frac{1}{2}>0\)