a.ta có: $\frac{-2019}{2020}$ =-1 +$\frac{1}{2020}$
=> $\frac{-2020}{2021}$ =-1 +$\frac{1}{2021}$
vậy $\frac{1}{2021}$<$\frac{1}{2020}$ nên $\frac{-2019}{2020}$ >$\frac{-2020}{2021}$
b)ta có: $\frac{84}{83}$ >0 mà $\frac{-337}{337}$ <0
=> $\frac{84}{83}$ > $\frac{-337}{337}$