`text{Đáp án + Giải thích các bước giải}`
`text{Ta có:}`
`A=3+3^2+3^3+3^4+3^5+3^6+...+3^19`
`=>A=3+(3^2+3^3)+(3^4+3^5)+(3^6+3^7)+...+(3^18+3^19)`
`=>A=3+3^2(1+3)+3^4(1+3)+3^6(1+3)+...+3^18(1+3)`
`=>A=3+3^{2}.4+3^{4}.4+3^{6}.4+...+3^{18}.4`
`=>A=3+(3^{2}.4+3^{4}.4+3^{6}.4+...+3^{18}.4)`
`=>A=3+4.(3^2+3^4+3^6+...+3^18)`
Do `4.(3^2+3^4+3^6+...+3^18)\vdots4` mà `+3` hay thừa `3` ra thì phép chia `A` cho `4` thì sẽ dư `3`
Vậy `A=3+3^2+3^3+3^4+3^5+...+3^19` chia cho `4` sẽ dư `3`
