a: \(A=x^2-x+\dfrac{1}{4}+\dfrac{7}{4}=\left(x-\dfrac{1}{2}\right)^2+\dfrac{7}{4}>=\dfrac{7}{4}\)
Dấu '=' xảy ra khi x=1/2
b: \(B=x^2+5x+\dfrac{25}{4}-\dfrac{25}{4}=\left(x+\dfrac{5}{2}\right)^2-\dfrac{25}{4}>=-\dfrac{25}{4}\)
Dấu '=' xảy ra khi x=-5/2