Đáp án:
`text{Đề : Tính B (x)}` `= x^5 - 2021 x^4 + 2021 x^3 - 2021x^2 + 2021x - 1000`
`text{Ta có : B (2020)}`
`-> x = 2020`
`-> x + 1 = 2021 (1)`
`text{Ta có : B (x)}` `= x^5 - 2021 x^4 + 2021 x^3 - 2021x^2 + 2021x - 1000`
`text{Thay (1) vào ta được :}`
`⇔ B (x) = x^5 - (x + 1) x^4 + (x + 1) x^3 - (x + 1) x^2 + (x + 1)x - 1000`
`⇔ B (x) = x^5 - x^5 - x^4 + x^4 + x^3 - x^3 - x^2 + x^2 + x - 1000`
`⇔ B (x) = (x^5 - x^5) + (-x^4 + x^4) + (x^3 - x^3) + (-x^2 + x^2) + (x - 1000)`
`⇔ B (x) = x -1000`
`⇔ B (2020) = 2020 - 1000`
`⇔ B (2020) = 1020`
`text{Vậy B (2020) = 1020}`
