\(-x^2+x+1=-\left(x^2-2\cdot x\cdot\dfrac{1}{2}+\dfrac{3}{4}\right)+\dfrac{7}{4}=-\left(x^2-\dfrac{1}{2}\right)^2+\dfrac{7}{4}\)
Vì: \(-\left(x-\dfrac{1}{2}\right)^2\le0\Rightarrow-\left(x-\dfrac{1}{2}\right)^2+\dfrac{7}{4}\le\dfrac{7}{4}\)
Vậy Amin = 7/4 khi x = 1/2