`a, A = 3+3^2+3^3+...+3^100`
`3A = 3^2 + 3^3 + 3^4+...+3^101`
`3A - A = 3^101 - 3`
`2A = 3^101-3`
`b,` Dựa theo câu a ta có :
` 2A = 3^101-3`
`2A + 3 = 3^201`
Vậy ` n = 201`
`c, A = 3+3^2+3^3+...+3^100 `
` A = (3++3^2+3^3)+ .... + (3^98+3^99+3^100)`
`A= ( 3.1+3.3+3.6) + ..... + (3^98.1+3^98.3+3^98.6)`
`A = [3. (1+3+6)] + ... + [3^98.(1+3+6)]`
`A = [3.10]+....+[3^98.10]`
`A = 10 . ( 3 + ...+3^98]` `vdots` `5`
Vậy ` A` `vdots` `5`
`A = 3+3^2+3^3+...+3^100 `
`A = 3 + 3^2 + 3^3 + ... + 3^100`
`A = (3 + 3^2 + 3^3 + 3^4) + ... + (3^97 + 3^98 + 3^99 + 3^100)`
`A = 120 + ... + 3^96 . 120`
Vì `120 ` `vdots` `12`
Vậy `A ` `vdots` `12`