c) $\left ( 4x + 2 \right ).\left ( x^{2} + 1 \right ) = 0$
$\Leftrightarrow 4x + 2 = 0$ (vì $x^{2} + 1 > 0$ với mọi $x$)
$\Leftrightarrow x = -\dfrac{1}{2}$
d) $\left ( 2x + 7 \right ).\left ( x - 5 \right ).\left ( 5x + 1 \right ) = 0$
$\Leftrightarrow \left[ \begin{array}{l}2x + 7 = 0\\x - 5 = 0\\5x + 1 = 0\end{array} \right.$
$\Leftrightarrow \left[ \begin{array}{l}x = -\dfrac{7}{2}\\x = 5\\x = -\dfrac{1}{5}\end{array} \right.$
