= 2 . ( ` 1/{ 1. 2 } ` + ` 1/{ 2 .3 } ` + ` 1/{ 3 .4} ` + ... + ` 1/{ 2020. 2021 } ` )
= 2 . ( 1 - ` 2 / 2 ` + ` 1 / 2 ` - ` 1 / 3 ` + ` 1 / 3 ` - ` 1 / 4 ` + ... + ` 1 / 2020 ` - ` 1 / 2021` )
= 2 . ` ( 1 - 1 / 2021 ) `
= 2 . ` 2020 / 2021 `
= ` 4040 / 4042 `
Chúc bạn học tốt !