Hướng dẫn:\(\sqrt{{{A}^{2}}}=\left| A \right|=\left\{ \begin{align} & A\,\text{nếu}\,A\ge 0 \\ & -A\,\text{nếu}\,A<0 \\ \end{align} \right.\)
a) \(2\sqrt{{{a}^{2}}}-5a=2\left| a \right|-5a=-2a-5a=-7a\) vì \(a<0\);
b) \(\sqrt{25{{a}^{2}}}+3a=\left| 5a \right|+3a=5a+3a=8a\) vì \(a\ge 0 \);
c) \(\sqrt{9{{a}^{4}}}+3{{a}^{2}}=\sqrt{{{\left( 3{{a}^{2}} \right)}^{2}}}+3{{a}^{2}}=3{{a}^{2}}+3{{a}^{2}}=6{{a}^{2}}\);
d) \(5\sqrt{4{{a}^{6}}}-3{{a}^{3}}=5\left| 2{{a}^{3}} \right|-3{{a}^{3}}=-10{{a}^{3}}-3{{a}^{3}}=-13{{a}^{3}}\) vì \(a<0 \).
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