C=2^1+2^2+2^3+...........+2^2013 =
nr termeni 2013
2013 divizibil cu 3 deci ii putem grupa cate 3
=2^1×(2^0+2^1+2^2)+...........2^2011×(2^0+2^1+2^2) =
=2×(1+2+4)+...........2^2011×(1+2+4) =
=2×7+...........2^2011×7=
=7×(2^1+...+2^2011) deci divizibil cu 7