Ta có: B=7+72+73+...+78
=(7+73)+(72+74)+...+(76+78)
=7(1+14)+72(1+14)+...+76(1+14)
=7.15+72.15+...+76.15
=15(7+72+...+76) chia hết cho 5
vậy B chia hết cho 5
\(B=7+7^2+7^3+...+7^8\)
\(B=7\left(1+7^2\right)+7^2\left(1+7^2\right)+7^3\left(1+7^2\right)+7^4\left(1+7^2\right)+7^5\left(1+7^2\right)+7^6\left(1+7^2\right)\)
\(B=7.15+7^2.15+...+7^6.15\)
\(B⋮15\Rightarrow B⋮3\)