Đáp án:
$\begin{array}{l}
a)6.{a^{6k}} - 4\\
= 6.{\left( {{a^{2k}}} \right)^3} - 4\\
= {6.5^3} - 4\\
= 6.125 - 4\\
= 750 - 4\\
= 746\\
b){\left( {5{x^2}{y^4}} \right)^3} + {\left( { - 7{y^3}{z^5}} \right)^2} = 0\\
\Leftrightarrow {\left( {5x{y^2}} \right)^{2.3}} + {\left( {7{y^3}{z^5}} \right)^2} = 0\\
\Leftrightarrow {\left( {125{x^3}{y^6}} \right)^2} + {\left( {7{y^3}{z^5}} \right)^2} = 0\\
Do:\left\{ \begin{array}{l}
{\left( {125{x^3}{y^6}} \right)^2} \ge 0\\
{\left( {7{y^3}{z^5}} \right)^2} \ge 0
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
125{x^3}{y^6} = 0\\
7{y^3}{z^5} = 0
\end{array} \right.\\
\Leftrightarrow \left\{ \begin{array}{l}
x = 0/y = 0\\
y = 0/z = 0
\end{array} \right.\\
Vậy\,x = y = z = 0
\end{array}$
