Đáp án:
$ MaxA=\dfrac{13}{4}$
$Min B=\dfrac{-21}{4}$
Giải thích các bước giải:
$\begin{split}A&=-4x^2+6x+1\\&=-((2x)^2-2.2x.\dfrac{3}{2}+\dfrac{9}{4})+\dfrac{13}{4}\\&=-(2x-\dfrac{3}{2})^2+\dfrac{13}{4}\\&\le \dfrac{13}{4} \end{split}$
$\rightarrow MaxA=\dfrac{13}{4}$
$\begin{split}B&=9x^2+3x-5\\&=(3x)^2-2.3x.\dfrac{1}{2}+\dfrac{1}{4}-\dfrac{21}{4}\\&=(3x-\dfrac{1}{2})^2-\dfrac{21}{4}\\&\ge -\dfrac{21}{4}\end{split}$
$\rightarrow Min B=\dfrac{-21}{4}$
