Đáp án:
$\begin{array}{l}
2\left( {x - y} \right)\left( {x + y} \right) + {\left( {x + y} \right)^2} + {\left( {x - y} \right)^2}\\
= {\left( {x + y} \right)^2} + 2\left( {x + y} \right)\left( {x - y} \right) + {\left( {x - y} \right)^2}\\
= {\left( {x + y + x - y} \right)^2}\\
= {\left( {2x} \right)^2}\\
= 4{x^2}\\
b)\\
{\left( {2x - 1} \right)^2} - 2{\left( {2x - 3} \right)^2} + 4\\
= 4{x^2} - 4x + 1 - 2\left( {4{x^2} - 2.2x.3 + 9} \right) + 4\\
= 4{x^2} - 4x + 1 - 8{x^2} + 24x - 18 + 4\\
= - 4{x^2} + 20x - 13\\
c)\\
{y^2} - 8y + 10\\
= {y^2} - 2.y.4 + 16 - 6\\
= {\left( {y - 4} \right)^2} - 6 \ge - 6\\
\Leftrightarrow GTNN = - 6\,khi:y = 4
\end{array}$
