Đáp án:
`A=3^1+3^2+3^3=...+3^2018+3^2019`
`A=3^1+3^2+3^3+(3^4+3^5+3^6+3^7)+(3^8+3^9+3^10+3^11)...+(3^2016+3^2017+3^2018+3^2019)`
`A=39+3^4.(1+3+3^2+3^3)+3^8.(1+3+3^2+3^3)+...+3^2016.(1+3+3^2+3^3)`
`A=39+3^4.40+3^8.40+...+3^2016.40`
`A=40.(3^4+3^8+...+3^2016)+39 : 4 dư 3`
Giải thích các bước giải: