Đáp án:
$\begin{array}{l}
s){81^{ - 2x}}{.27^x} = {9^5}\\
\Leftrightarrow {\left( {{3^4}} \right)^{ - 2x}}.{\left( {{3^3}} \right)^x} = {\left( {{3^2}} \right)^5}\\
\Leftrightarrow {3^{ - 8x}}{.3^{3x}} = {3^{10}}\\
\Leftrightarrow - 8x + 3x = 10\\
\Leftrightarrow - 5x = 10\\
\Leftrightarrow x = - 2\\
Vậy\,x = - 2\\
q){7^{2 + x}} + {2.7^{x - 1}} = 345\\
\Leftrightarrow {7^{x - 1}}{.7^3} + {2.7^{x - 1}} = 345\\
\Leftrightarrow {7^{x - 1}}.\left( {{7^3} + 2} \right) = 345\\
\Leftrightarrow {7^{x - 1}}.345 = 345\\
\Leftrightarrow {7^{x - 1}} = 1\\
\Leftrightarrow x - 1 = 0\\
\Leftrightarrow x = 1\\
Vậy\,x = 1
\end{array}$
