B=(1×2021+2×2020+3×2019+....+2021×1):(1×2+2×3+3×4+....+2021×2022)

Đặt A=1×2021+2×2020+3×2019+....+2021×1

      A=2021+2020+2020+2019+2019+2019+.....+2021

      A=1+(1+2)+(1+2+3)+....+(1+2+3+.....+2021)

      A=(0+1)/2×2+(1+2)/2×2+......+(1+2021)/2×2021

      A=1×2/2+2×3/2+......+2021×2022/2

      A=1×2+2×3+3×4+......+2021×2022/2

⇒B=A:1×2+2×3+3×4+....+2021×2022

⇒B=1/2

Vậy B=1/2