giúp tôi với các bác

`2021/( 1 xx 2) + 2021/( 2 xx 3) + 2021/( 3 xx 4) + ... + 2021/( 1999 xx 2000)`

`= 2021 . (1/( 1 xx 2) + 1/( 2 xx 3) + 1/( 3 xx 4) + ... +1/( 1999 xx 2000))`

`= 2021 . ( 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/1999 - 1/2000)`

`= 2021 . ( 1 - 1/2000)`

`= 2021 . 1999/2000`

`=4039979/2000`