Tính `A(2018) => x = 2018 => x- 2018 =0`
Ta có:
`A(x)= x^21 - 2019x^20 + 2019x^19 -....-2019x^2 + 2019x -1`
`A(x)= x^21 - (2018+1)x^20 + (2018+1)x^19 -...-(2018+1)x^2 + (2018+1)x -1`
`A(x)= x^21 - 2018x^20 - x^20 + 2018x^19 + x^19 -...-2018x^2 - x^2 + 2018x + x-1`
`A(x)= x^20(x-2018) -x^19(x -2018) +....- x(x -2018) + x-1`
`A(x)= x^20 . 0 - x^19 .0 +...- x.0 + x-1`
`A(x)= x-1`
`=> A(2018) = 2018 -1`
`=> A(2018)= 2017`
Vậy `A(2018)= 2017`
