Đáp án:
$\begin{array}{l}
b)g\left( x \right) = 0\\
\Rightarrow - 5{x^2} + 30x = 0\\
\Rightarrow 5x.\left( { - x + 6} \right) = 0\\
\Rightarrow \left[ \begin{array}{l}
x = 0\\
- x + 6 = 0
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = 0\\
x = 6
\end{array} \right.\\
c)h\left( x \right) = 0\\
\Rightarrow \left( {x - 3} \right)\left( {16 - 4x} \right) = 0\\
\Rightarrow \left[ \begin{array}{l}
x = 3\\
x = 4
\end{array} \right.\\
d)k\left( x \right) = 2{x^3} - 32x = 0\\
\Rightarrow 2.x\left( {{x^2} - 16} \right) = 0\\
\Rightarrow \left[ \begin{array}{l}
x = 0\\
{x^2} = 16
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = 0\\
x = 4\\
x = - 4
\end{array} \right.\\
e)m\left( x \right) = {x^2} + 7x - 8 = 0\\
\Rightarrow {x^2} + 8x - x - 8 = 0\\
\Rightarrow x\left( {x + 8} \right) - \left( {x + 8} \right) = 0\\
\Rightarrow \left( {x + 8} \right)\left( {x - 1} \right) = 0\\
\Rightarrow \left[ \begin{array}{l}
x = - 8\\
x = 1
\end{array} \right.\\
f)n\left( x \right) = 5{x^2} + 9x + 4 = 0\\
\Rightarrow 5{x^2} + 5x + 4x + 4 = 0\\
\Rightarrow \left( {x + 1} \right)\left( {5x + 4} \right) = 0\\
\Rightarrow \left[ \begin{array}{l}
x = - 1\\
x = - \frac{4}{5}
\end{array} \right.
\end{array}$
