Đáp án:
Giải thích các bước giải:
`A= 2^2021-2^2020- 2^2019-...-2-1`
`=2^(2021)-(1+2+...+2^(2019)+2^(2020))`
Đặt `(1+2+...+2^(2019)+2^(2020)=B`
`=> 2B=2+2^2+...+2^(2021)`
`=> 2B-B=2^(2021)-1`
`=> B=2^(2021)-1`
`=> A=2^(2021)-(2^(2021)-1)`
`=2^(2021)-2^(2021)+1`
`=1`
Vậy `A=1`
`------`
`B=2^100-(2^98+2^96+2^94+...+2^2).3`
Đặt `C=2^98+2^96+2^94+...+2^2`
`2^2C=2^100+2^98+2^96+...+2^4`
`2^2C-C=2^100-2^2`
`3C=2^100-4`
`C=(2^100-4)/3`
`=> B=2^100-(2^100-4)/3 . 3`
`=2^100-(2^100-4)`
`=4`
Vậy `B=4`
