Răspuns:
S = 2020/2021
Explicație pas cu pas:
S = 1/1×2 + 1/2×3 + 1/3×4 + .. + 1/2020×2021
putem rescrie relatia astfel
S = (1/1 - 1/2) + (1/2 - 1/3) + .. + (1/2020 - 1/2021)
reducem
S = 1/1 - 1/2021
S = 2021/2021 - 1/2021
