Đáp án:
$\begin{array}{l}
a)\sqrt {0,49} = 0,7\\
b)\sqrt {{a^4}{{\left( {a - 2} \right)}^2}} = {a^2}.\left| {a - 2} \right|\\
= \left[ \begin{array}{l}
{a^2}\left( {a - 2} \right) = {a^3} - 2a\left( {khi:a \ge 2} \right)\\
2a - {a^3}\left( {khi:0 < a < 2} \right)
\end{array} \right.\\
c)\sqrt {25{{\left( {7 - a} \right)}^2}} = 5.\left( {a - 7} \right) = 5a - 35\left( {khi:a \ge 7} \right)\\
d)\dfrac{1}{{a - 3b}}.\sqrt {{a^6}.{{\left( {a - 3b} \right)}^2}} \\
= \dfrac{1}{{a - 3b}}.{a^3}.\left( {a - 3b} \right)\left( {do:a > 3b > 0} \right)\\
= {a^3}
\end{array}$
