P=(7+7^2+7^3+7^4)+(7^5+7^6+7^7+7^8)+........+(7^2013+7^2014+7^2015+7^2016)
P=7.(1+7+7^2+7^3)+7^5.(1+7+7^2+7^3)+.........+7^2013.(1+7+7^2+7^3)
P=7.400+7^5.400+..........+7^2013.400
P=400.(7+7^5+......+7^2013)
P=20^2.(7+7^5+......+7^2013) chia hết cho 20^2
Vậy.......
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