Question

a=2^36+2^35+...+2^2+2se divide cu 15

Answer 1

Grupezi termenii cate 4 si dai factor comun 
2(1+2+2^2+2^3)+ 2^5(1+2+2^2+2^3) +......+2^33(1+2+2^2+2^3)=
2(1+2+4+8)+2^5(1+2+4+8)+....+2^33(1+2+4+8)=
15*2+15*2^5+.....+15*2^33=
15*(2+2^5+....+2^33) divizibil cu 15