`1/(2^2) + 1/(3^2) + 1/(4^2) + ............. + 1/(2021^2)`
`= 1/(2.2) + 1/(3.3) + 1/(4.4) + ............ + 1/(2021.2021)`
`< 1/4 + 1/(2.3) + 1/(3.4) + ........... + 1/(2020.2021)`
`< 1/4 + 1/2 - 1/3 + 1/3 - 1/4 + .................. + 1/2020 - 1/2021`
`< 3/4 - 1/2021 < 3/4`
`⇒ 1/(2^2) + 1/(3^2) + 1/(4^2) + ............. + 1/(2021^2) < 3/4`