đề là c/m hả bạn
\(A=x^2-3x+3\)
\(A=x^2-2\cdot x\cdot\dfrac{3}{2}+\left(\dfrac{3}{2}\right)^2+\dfrac{3}{4}\)
\(A=\left(x-\dfrac{3}{2}\right)^2+\dfrac{3}{4}\)
Vì \(\left(x-\dfrac{3}{2}\right)^2\ge0\forall x\)
\(\Rightarrow A\ge\dfrac{3}{4}>0\forall x\)
Đặt \(A=x^2-3x+3\)
\(\Leftrightarrow A=x^2-2.x.\frac{3}{2}+\frac{9}{4}-\frac{9}{4}+3\)
\(\Leftrightarrow A=\left(x-\frac{3}{2}\right)^2+\frac{3}{4}>0\)
=> Điều phải chứng minh