=3^x.3 + 3^x.3^2 + 3^x.3^3 +...+ 3^x.3^100
=3^x . ( 3+3^2+3^3+3^4+...+3^100)
=3^x .( (3+3^2+3^3+3^4)+(3^5+3^6+3^7+3^8)+...+ (3^97+3^98+3^99+3^100)
=3^x . ( 120 + 3^4 .(3+3^2+3^3+3^4) +...+ 3^96 (3+3^2+3^3+3^4)
=3^x . ( 120+ 3^4. 120+...+3^96.120)
=3^x . 120 . (1+3^4+...+3^96)
chia hết cho 120( đây là cách giải lớp 6)