`= 1/2020 - [ 1/(2020.2021) +1/(2021.2002)]`
`= 1/2020 - 1/(2020.2021) - 1/(2021.2002)`
`= 1/2020 - 1/{2020} .{1}/2021 -1/(2021.2002)`
`= 1/{2020}. (1-1/2021) -1/(2021.2002)`
`=1/{2020} .2020/2021 -1/(2021.2002)`
`= 1/{2021} - 1/{2021}. {1}/{2002}`
`= 1/{2021}.(1-1/2002)`
`=1/{2021}.2001/{2002}`
`=2001/(2021.2002)`