Đáp án:
Giải thích các bước giải:
Bài 4:
`a, A = 2 + 2^2 +...........+ 2^2020`
`A = ( 2 + 2^2 ) + ( 2^3 + 2^4 ) +.........+ ( 2^2019 + 2^2020 )`
`A = 6 + 2^2 . ( 2 + 4 ) +.........+ 2^2018 . ( 2 + 4 )`
`A = 6 + 2^2 . 6 +...........+ 2^2018 . 6`
`A = 6 . ( 2^2 +..............+ 2^2018 ) vdots 6`
Vậy `A vdots 6`
`b, A = 2 + 2^2 +...........+ 2^2020`
`A = ( 2 + 2^2 + 2^3 ) +.............+ ( 2^2018 + 2^2019 + 2^2020 )`
`A = 2 . ( 1 + 2 + 4 ) +............+ 2^2018 . ( 1 + 2 + 4 )`
`A = 2 . 7 +...........+ 2^2018 . 7`
`A = 7 . ( 2 +...........+ 2^2018 ) vdots 7`
Vậy `A vdots 7`
`c, A = 2^2 + 2^3 +..............+ 2^2020`
`A = ( 2^2 + 2^3 + 2^4 + 2^5 ) +...........+ ( 2^2017 + 2^2018 + 2^2019 + 2^2020 )`
`A = 2 . ( 2 + 4 + 8 + 16 ) +..........+ 2^2016 . ( 2 + 4 + 8 + 16 )`
`A = 2 . 30 +..........+ 2^2016 . 30`
`A = 30 . ( 2 +............+ 2^2016 ) vdots 30`
Vậy `A vdots 30`
