`Đặt A = 1/(2^2) + 1/(3^2) + ............ + 1/(2021^2)`
`A < 1/4 + 1/(2.3) + .................. + 1/(2020.2021)`
`A < 1/4 + (3 - 2)/(2.3) + .................. + (2021 - 2020)/(2020 . 2021)`
`A < 1/4 + 1/2 - 1/3 + ................ + 1/2020 - 1/2021`
`A < 3/4 - 1/2021 < 3/4`
`⇒ A < 3/4`