`1 + 1/3 + 1/6 + 1/10 +...+ 2/(x(x+1)) = 4040/2021`
`⇔ 2 . (1/2 + 1/6 + 1/12 + 1/20 +...+ 1/(x(x+1))) = 4040/2021`
`⇔ 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 +...+ 1/(x(x+1)) = 2020/2021`
`⇔ 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 +...+ 1/x - 1/(x+1) = 2020/2021`
`⇔ 1 - 1/(x+1) = 2020/2021`
`⇔ 1/(x+1) = 1/2021`
`⇔ x + 1 = 2021 ⇔ x = 2020`