Ta có :
n2020 - n2019 + n - 2019 = n2019 ( n - 1 ) + ( n - 1 ) - 2018
= ( n - 1 ) ( n2019 + 1 ) - 2018 ⋮ n - 1
Mà ( n - 1 ) ( n2019 + 1 ) ⋮ n - 1
⇒ 2018 ⋮ n - 1 ⇔ n - 1 ϵ Ư(2018)
Ta có :
n2020 - n2019 + n - 2019 = n2019 ( n - 1 ) + ( n - 1 ) - 2018
= ( n - 1 ) ( n2019 + 1 ) - 2018 ⋮ n - 1
Mà ( n - 1 ) ( n2019 + 1 ) ⋮ n - 1
⇒ 2018 ⋮ n - 1 ⇔ n - 1 ϵ Ư(2018)