A=1+5+5^2+..+5^9/1+5+5^2+...+5^8
=1+5^9/1+5+5^2+...+5^8
B=1+3+3^2+..+3^9/1+3+3^2+..+3^8
=1+3^9/1+3+3^2+..+3^8
đặt A' =1+5+5^2+...+5^8
5A'=5+5^2+5^3+...+5^9
5A'-A'=5+5^2+5^3+...+5^9-5-1-5-5^2-...-5^8
4A'=5^9-1=>A'=(5^9-1):4
tương tự B'=(3^9-1):4
A=1+5^9/(5^9-1)/4=4.5^9/5^9-1
B=1+3^9/(3^9-1)/4=4.3^9/3^9-1
=> A<B