Sửa `31 -> 13`.
`A = (1+3+3^2) + (3^3+3^4+3^5) + ... + (3^99+3^100+3^101)`.
`= 13 + 3^3 .13 + ... + 3^99 . 13.`
`= 13 . (1 + 3^3 + ... + 3^99)`
Sửa 31→1331→13.
A=(1+3+32)+(33+34+35)+...+(399+3100+3101)A=(1+3+32)+(33+34+35)+...+(399+3100+3101).
=13+33.13+...+399.13.=13+33.13+...+399.13.
=13.(1+33+...+399)