Giải thích các bước giải:
\(\begin{array}{l}
C1.\\
a)\,4{x^2} - 49 = {\left( {2x} \right)^2} - {7^2} = \left( {2x - 7} \right)\left( {2x + 7} \right)\\
b)\,{x^2} + 4x + 4 - 25{x^2}\\
= {\left( {x + 2} \right)^2} - {\left( {5x} \right)^2}\\
= \left( {x + 2 + 5x} \right)\left( {x + 2 - 5x} \right)\\
= \left( {6x + 2} \right)\left( {2 - 4x} \right)\\
c)\,{x^3} + 3{x^2} - x - 3\\
= {x^2}\left( {x + 3} \right) - \left( {x + 3} \right)\\
= \left( {{x^2} - 1} \right)\left( {x + 3} \right)\\
= \left( {x - 1} \right)\left( {x + 1} \right)\left( {x + 3} \right)
\end{array}\)
