Tìm x: \({3^{x - 1}} = \frac{1}{{243}}\).

\[\begin{array}{l}{3^{x - 1}} = \frac{1}{{243}}\\ \Rightarrow {3^{x - 1}} = {3^{ - 5}}\\ \Rightarrow x - 1 = - 5\\ \Rightarrow x = - 5 + 1\\ \Rightarrow x = - 4\end{array}\]

Đáp số: x = \[ - 4\].