Ta có : \(5x-x^2+13=-x^2+5x+13\)
\(=-\left(x^2-5x-13\right)\)
\(=-\left[x^2-2.x.\dfrac{5}{2}+\left(\dfrac{5}{2}\right)^2-\dfrac{25}{4}-13\right]\)
\(=-\left[\left(x-\dfrac{5}{2}\right)^2-\dfrac{77}{4}\right]\)
\(=-\left(x-\dfrac{5}{2}\right)^2+\dfrac{77}{4}\)
Do \(-\left(x-\dfrac{5}{2}\right)^2\le0\) với mọi x (dấu "=" xảy ra \(\Leftrightarrow x-\dfrac{5}{2}=0\Rightarrow x=\dfrac{5}{2}\))
\(\Rightarrow-\left(x-\dfrac{5}{2}\right)^2+\dfrac{77}{4}\le\dfrac{77}{4}\) hay \(A\le0\) (dấu "=" xảy ra \(\Leftrightarrow x=\dfrac{5}{2}\))
Vậy Max A=\(\dfrac{77}{4}\) tại x=\(\dfrac{5}{2}\)