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Đặt A=2004+20042+20043+...+200410
=(2004+20042)+(20043+20044)+...+(20049+200410)
=2004(1+2004)+20043(1+2004)+...+20049(1+2004)
=2004.2005+20043.2005+...+20049.2005 chia hết cho 2005
Vậy A chia hết cho 2005.
Đặt A=2004+20042+20043+...+300410
A=(2004+20042)+(20043+20044)+.....+(20049+200410)
A=(2004+20042)+20042.(2004+20042)+...+20048.(2004+20042)
A=4018020+20042.4018020+.....+20048.4018020
A=4018020.(1+20042+....+20048)
A=2004.2005.(1+20042+...+20048)\(⋮\)2005
Vậy A\(⋮\)2005
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