*>Ta có:
[tex]A= 2^9 +2^{99}=2^2(2^7 + 2^{97})=4(2^7 + 2^{97}) \equiv 0 [/tex](mod 4).
*> [tex]2^5 = 32 \equiv [/tex] 7 (mod 25)
\Rightarrow [tex]2^{10} \equiv 7^2[/tex] (mod 25)
\Rightarrow [tex]2^{10} \equiv -1[/tex] (mod 25)
Mặt khác:
A=[tex] 2^9 +2^{99} =2^9(1+2^{90})[/tex]
Mà [tex](1+2^{90}) = 1 + (2^{10})^9 \equiv 1 +(-1) \equiv 0[/tex] (mod 25)
\Rightarrow [tex]2^9 +2^{99} \equiv 0 [/tex](mod 25)
(4;25)=1
\Rightarrow [tex]A \equiv 0 [/tex](mod 100)
\Rightarrow A chia hết cho 100.