\(=>A=\left(3+3^2+3^3\right)+\left(3^4+3^5+3^6\right)+.....+\left(3^{97}+3^{98}+3^{99}\right)\)
\(=>A=3\left(1+3+9\right)+3^4\left(1+3+9\right)+......+3^{97}\left(1+3+9\right)\)
\(=>A=13.3+13.3^4+......+13.3^{97}\)
\(=>A=13\left(3+3^4+3^7+.......+3^{97}\right)⋮13\)
Vậy A chia hết cho 13;
CHÚC BẠN HỌC TỐT..............