Ta có:
`|x-2020| ≥ 0` mọi `x ∈ Q`
`|y-2021| ≥ 0` mọi `y ∈ Q`
`=> |x-2020| + |y-2021| ≥ 0` mọi `x,y ∈ Q`
Dấu = có khi:
$\left \{ {{|x-2020|=0} \atop {|y-2021|=0}} \right.$
=> $\left \{ {{x-2020=0} \atop {y-2021=0}} \right.$
=> $\left \{ {{x=2020} \atop {y=2021}} \right.$
Vậy `x = 2020; y = 2021`