Đáp án:
$\begin{array}{l}
1)\,a)\\
{15^{15}} = {\left( {3.5} \right)^{15}} = {3^{15}}{.5^{15}}\\
{81^3} = {\left( {{3^5}} \right)^3} = {3^{15}}\\
{125^5} = {\left( {{5^3}} \right)^5} = {5^{15}}\\
Do:\,{3^{15}} < {5^{15}} < {3^{15}}{.5^{15}}\\
\Rightarrow {81^3} < {125^5} < {15^{15}}\\
2)a)\frac{{121}}{{11}} - \frac{{4x + 5}}{3} = 4\\
\Rightarrow 11 - \frac{{4x + 5}}{3} = 4\\
\Rightarrow \frac{{4x + 5}}{3} = 11 - 4 = 7\\
\Rightarrow 4x + 5 = 7.3\\
\Rightarrow 4x + 5 = 21\\
\Rightarrow 4x = 21 - 5 = 16\\
\Rightarrow x = 16:4 = 4\\
Vậy\,x = 4
\end{array}$