Đáp án + Giải thích các bước giải:
`A=1+2+2^2+2^3+...+2^59+2^56`
`=1+(2+2^2+2^3)+(2^4+2^5+2^6)+...+(2^58+2^59+2^60)`
`=1+2(1+2+2^2)+2^4(1+2+2^2)+...+2^58(1+2+2^2)`
`=1+2·7+2^4·7+...+2^58·7`
`=7(2+2^4+...+2^58)+1`
`\text{Vì} 7(2+2^4+...+2^58)\vdots7`
`=>A \text{chia} 7 \text{dư} 1`