2+2^2+2^3+...+2^1000 =(2+2^2+2^3+2^4)+(2^5+2^6+2^7+2^8)+...+(2^97+2^98+2^99+2^100)
=2.(1+2+2^2+2^3)+2^5.(1+2+2^2+2^3)+...+2^97.(1+2+2^2+2^3)
=2.31+2^5.31+...+2^97.31
=31.(2+2^5+...+2^97) chia hết cho 31
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