8+8^2+8^3=8(1+8+8^2)= 8•(9+64)=8•73 numar care-i divizibil clar cu 73.
Suma 8+8^2+....+8^42 are 42 termeni, iar 42:3=14 deci putem grupa termenii sumei trei cate trei, astfel rezultand 14 triplete de termeni consecutivi:
8+8^2+...+8^42=(8+8^2+8^3)+(8^4+8^5+8^6)+...+(8^40+8^41+8^42)= (8+8^2+8^3)+ 8^3(8+8^2+8^3)+...+8^39(8+8^2+8^3)= (8+8^2+8^3)(1+8^3+...+8^39)=73•8•(1+8^3+..+8^39) divizibil clar cu 73.
