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`(k+1)/2021 + (k+2)/2020 = (k+3)/2019 + (k+4)/2018`
`-> (k+1)/2021 + (k+2)/2020 + 2 = (k+3)/2019 + (k+4)/2018 + 2`
`-> ( (k+1)/2021 +1) + ( (k+2)/2020 + 1) = ( (k+3)/2019 + 1) + ( (k+4)/2018 + 1)`
`-> ( (k+1)/2021 + 2021/2021) + ( (k+2)/2020 + 2020/2020) = ( (k+3)/2019 + 2019/2019) + ( (k+4)/2018 + 2018/2018)`
`-> (k+1+2021)/2021 + (k+2+2020)/2020 = (k+3+2019)/2019 + (k+4+2018)/2018`
`-> (k+2022)/2021 + (k+2022)/2020 = (k+2022)/2019 + (k+2022)/2018`
`-> (k+2022)/2021 + (k+2022)/2020 - (k+2022)/2019 - (k+2022)/2018 =0`
`-> (k+2022) (1/2021 + 1/2020 - 1/2019 - 1/2018)=0`
`-> k+2022=0` (Do `1/2021 + 1/2020 - 1/2019 - 1/2018 \ne 0`)
`->k=0-2022`
`-> k=-2022`
Vậy `k=-2022`