Ta có: C = 2019^2018+1/2019^2019+1
=> 2019C = 2019^2019+2019/2019^2019+1
= 2019^2019+1+2018/2019^2019+1
= 2019^2019+1/2019^2019+1 + 2018/2019^2019+1
= 1 + 2018/2019^2019+1
Lại có: D = 2019^2019+1/2019^2020+1
=> 2019D = 2019^2020+2019/2019^2020+1
= 2019^2020+1+2018/2019^2020+1
= 2019^2020+1/2019^2020+1 + 2018/2019^2020+1
= 1 + 2018/2019^2020+1
Vì 2018/2019^2019+1 > 2018/2019^2020+1
=> 1 + 2018/2019^2019+1 > 1 + 2018/2019^2020+1
hay 2019C > 2019D
=> C > D
Vậy C > D.
Chúc bạn học tốt!