Đáp án:
$\begin{array}{l}
a)Thay\,x = \frac{1}{2};y = 2\\
\Rightarrow A = - {\left( {\frac{1}{2}} \right)^2} + 4.\frac{1}{2}.2 - 3\\
= - \frac{1}{4} + 4 - 3\\
= - \frac{1}{4} + 1\\
= \frac{{ - 1 + 4}}{4} = \frac{3}{4}\\
b)x + y + z = 0\\
\Rightarrow \left\{ \begin{array}{l}
x + y = - z\\
y + z = - x\\
x + z = - y
\end{array} \right.\\
B = \left( {x + y} \right)\left( {y + z} \right)\left( {z + x} \right)\\
= \left( { - z} \right).\left( { - x} \right).\left( { - y} \right)\\
= - \left( {x.y.z} \right)\\
= - 2
\end{array}$
