Đáp án: B=8
Giải thích các bước giải:
$\begin{array}{l}
\left\{ \begin{array}{l}
x - y - z = 0\\
x + 2y - 10z = 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
x = y + z\\
\left( {y + z} \right) + 2y - 10z = 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
x = y + z\\
3y - 9z = 0
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
x = y + z\\
y = 3z
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
x = 4z\\
y = 3z
\end{array} \right.\\
B = \frac{{2{x^2} + 4xy}}{{{y^2} + {z^2}}} = \frac{{2{{\left( {4z} \right)}^2} + 4.4z.3z}}{{{{\left( {3z} \right)}^2} + {z^2}}}\\
= \frac{{32{z^2} + 48{z^2}}}{{9{z^2} + {z^2}}} = \frac{{80{z^2}}}{{10{z^2}}}\\
= 8
\end{array}$
