Bài 1. Ta có : \(25 = {5^2};625 = {5^4} \Rightarrow {5^{2}} < {5^x} < {5^4}\)
Vậy \(x ∈ \{2; 3;4 \}\)
Bài 2. Ta có :
\({10^{20}} = \underbrace {{{10}^2}{{.10}^2}{{.....10}^2}}_{10} = {100^{10}} > {90^{10}}\)
Vậy \({10^{20}} >{9^{10}}\)
Bài 3. Ta có : \({2^n} + {4.2^n} = {5.2^{5}} \Rightarrow {2^n}\left( 1 + 4\right) = {5.2^5}\)
\(\Rightarrow {5.2^n} = {\rm{ }}{5.2^5} \Rightarrow {2^{n}} = {2^5} \Rightarrow n = 5.\)
