Đáp án:
$\begin{array}{l}
1)A = - {x^2} - 4x + 5\\
= - \left( {{x^2} + 4x} \right) + 5\\
= - \left( {{x^2} + 4x + 4} \right) + 4 + 5\\
= - {\left( {x + 2} \right)^2} + 9 \le 9\\
\Leftrightarrow GTLN:A = 9\,khi:x = - 2\\
2)A = - {x^2} + 4x + 12\\
= - \left( {{x^2} - 4x + 4} \right) + 4 + 12\\
= - {\left( {x - 2} \right)^2} + 16 \le 16\\
\Leftrightarrow GTLN:A = 16\,khi:x = 2\\
3)A = - {x^2} + 2x - 3\\
= - \left( {{x^2} - 2x + 1} \right) + 1 - 3\\
= - {\left( {x - 1} \right)^2} - 2 \le - 2\\
\Leftrightarrow GTLN:A = - 2\,khi:x = 1
\end{array}$
