2005+20052+20053+...+200510
=2005.(1+2005)+20053.(1+2005)+...+20059.(1+2005)
=2005.2006+20053.2006+...+20059.2006
=2006.(2005+20053+...+20059)
=>2005+20052+20053+...+200510 chia hết cho 2006
= 2005.(1+2005)+20052.(1+2005)+...+20059.(1+2005)
= 2006.(2005+20052+...+20059)
=> tổng trên chia hết cho 2006
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