Câu 15: a: Ta có: \(9< 3^n< 81\)\(\Leftrightarrow2< n< 4\)hay n=3b: Ta có: \(25\le5^n\le125\)\(\Leftrightarrow2\le n\le3\)hay \(n\in\left\{2;3\right\}\)Câu trả lời: Câu 15: a: Ta có: \(9< 3^n< 81\)\(\Leftrightarrow2< n< 4\)hay n=3b: Ta có: \(25\le5^n\le125\)\(\Leftrightarrow2\le n\le3\)hay \(n\in\left\{2;3\right\}\)