Đáp án:
$\begin{array}{l}
a){9^{x - 1}} = 81\\
\Rightarrow {9^{x - 1}} = {9^2}\\
\Rightarrow x - 1 = 2\\
\Rightarrow x = 2 + 1\\
\Rightarrow x = 3\\
\text{Vậy}\,x = 3\\
b)\left( {19x + {{2.5}^2}} \right):14 = {\left( {13 - 8} \right)^2} - {4^2}\\
\Rightarrow \left( {19x + 2.25} \right):14 = {5^2} - 16\\
\Rightarrow \left( {19x + 50} \right):14 = 9\\
\Rightarrow 19x + 50 = 9.14\\
\Rightarrow 19x = 126 - 50\\
\Rightarrow 19x = 76\\
\Rightarrow x = 4\\
\text{Vậy}\,x = 4\\
c){2.3^x} = {10.3^{12}} + {8.27^4}\\
\Rightarrow {2.3^x} = {10.3^{12}} + {8.3^{3.4}}\\
\Rightarrow {2.3^x} = {10.3^{12}} + {8.3^{12}}\\
\Rightarrow {2.3^x} = {3^{12}}.\left( {10 + 8} \right)\\
\Rightarrow {2.3^x} = {3^{12}}.18\\
\Rightarrow {2.3^x} = {3^{12}}{.3^2}.2\\
\Rightarrow {2.3^x} = {2.3^{14}}\\
\Rightarrow {3^x} = {3^{14}}\\
\Rightarrow x = 14\\
\text{Vậy}\,x = 14
\end{array}$
