Đáp án:
$\begin{array}{l}
3A\\
a)Dkxd:4x + 1 - 2\left( {5 - 2x} \right) \ne 0\\
\Leftrightarrow 4x + 1 - 10 + 4x \ne 0\\
\Leftrightarrow 8x \ne 9\\
\Leftrightarrow x \ne \frac{9}{8}\\
b)DKxd:2x\left( {x + 2} \right) - \left( {x + 1} \right)\left( {2x + 4} \right) \ne 0\\
\Leftrightarrow 2x\left( {x + 2} \right) - \left( {x + 1} \right).2.\left( {x + 2} \right) \ne 0\\
\Leftrightarrow \left( {x + 2} \right)\left( {2x - 2x - 2} \right) \ne 0\\
\Leftrightarrow \left( {x + 2} \right).\left( { - 2} \right) \ne 0\\
\Leftrightarrow x \ne - 2\\
3B\\
a)Dkxd: - x\left( {2x + 1} \right) + 2x\left( {1 + x} \right) \ne 0\\
\Leftrightarrow - 2{x^2} - x + 2x + 2{x^2} \ne 0\\
\Leftrightarrow x \ne 0\\
b)DKxd:x\left( {x + 2} \right) - \left( {x + 1} \right)\left( {x + 2} \right) \ne 0\\
\Leftrightarrow \left( {x + 2} \right)\left( {x - x - 1} \right) \ne 0\\
\Leftrightarrow \left( {x + 2} \right).\left( { - 1} \right) \ne 0\\
\Leftrightarrow x \ne - 2
\end{array}$
