Giải thích các bước giải:
Ta có:
\(\begin{array}{l}
a,\\
{84^{11}} > {81^{11}} = {\left( {{9^2}} \right)^{11}} = {9^{22}} > {9^{21}}\\
\Rightarrow - {84^{11}} < - {9^{21}}\\
\Leftrightarrow {\left( { - 84} \right)^{11}} < {\left( { - 9} \right)^{21}}\\
b,\\
{8^7} = {\left( {{2^3}} \right)^7} = {2^{21}} > {2^{20}} = {\left( {{2^4}} \right)^5} = {16^5}\\
\Rightarrow \dfrac{1}{{{8^7}}} < \dfrac{1}{{{{16}^5}}}\\
\Leftrightarrow {\left( {\dfrac{1}{8}} \right)^7} < {\left( {\dfrac{1}{{16}}} \right)^5}\\
\Leftrightarrow - {\left( {\dfrac{1}{8}} \right)^7} > - {\left( {\dfrac{1}{{16}}} \right)^5}\\
\Leftrightarrow {\left( { - \dfrac{1}{8}} \right)^7} > {\left( { - \dfrac{1}{{16}}} \right)^5}
\end{array}\)
