a)
\(9^{1005}=\left(3^2\right)^{1005}=3^{2010}\)
Vì \(2009< 2010\)
Nên \(3^{2009}< 3^{2010}\)
Vậy \(3^{2009}< 9^{1005}\)
b)
\(99^{20}=\left(99^2\right)^{10}=9801^{10}\)
Vì \(9801< 9999\)
Nên \(9801^{10}< 9999^{10}\)
Vậy \(99^{20}< 9999^{10}\)