Đáp án:
$\begin{array}{l}
a){\left( {2x + 1} \right)^2} - {\left( {x - 1} \right)^2}\\
= \left( {2x + 1 - x + 1} \right)\left( {2x + 1 + x - 1} \right)\\
= 3x.\left( {x + 2} \right)\\
b)kiem\,tra\,lai\,de\,nhe\\
c)\,49{\left( {y - 4} \right)^2} - 9{\left( {y + 2} \right)^2}\\
= {\left[ {7\left( {y - 4} \right)} \right]^2} - {\left[ {3\left( {y + 2} \right)} \right]^2}\\
= {\left( {7y - 28} \right)^2} - {\left( {3y + 6} \right)^2}\\
= \left( {7y - 28 - 3y - 6} \right)\left( {7y - 28 + 3y + 6} \right)\\
= \left( {4y - 34} \right)\left( {10y - 22} \right)
\end{array}$
