Ta có:
$\frac{2020}{2019}$ = $\frac{2019+1}{2019}$ = $\frac{2019}{2019}$ + $\frac{1}{2019}$ = 1 + $\frac{1}{2019}$
$\frac{2019}{2018}$ = $\frac{2018+1}{2018}$ = $\frac{2018}{2018}$ + $\frac{1}{2018}$ = 1 + $\frac{1}{2018}$
Vì 1 = 1 và $\frac{1}{2019}$ < $\frac{1}{2018}$
=> $\frac{2020}{2019}$ < $\frac{2019}{2018}$
Chúc học tốt!!!