a) \(3^{-8}\le3^x\le\frac{1}{243}.\)
\(\Rightarrow3^{-8}\le3^x\le3^{-5}\)
\(\Rightarrow-8\le x\le-5\)
\(\Rightarrow\left[{}\begin{matrix}x=-8\\x=-7\\x=-6\\x=-5\end{matrix}\right.\)
Vậy \(x\in\left\{-8;-7;-6;-5\right\}.\)
b) \(2^{-x}< \frac{1}{64}\)
\(\Rightarrow2^{-x}< 2^{-6}\)
\(\Rightarrow-x< -6\)
\(\Rightarrow x< 6.\)
Vậy \(x< 6.\)
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