tính nhanh ạ vvvvvvvvvvvvvvvvvvvvvvvvvvv

Đáp án + Giải thích các bước giải:

`D = 3/5 + 3/45 + 3/117 + 3/221 + 3/257`

`D = 3/{1xx5} + 3/{5xx9} + 3/{9xx13} + 3/{13xx17} + 3/{17xx21}`

`D = 3 xx ( 1/{1xx5} + 1/{5xx9} + 1/{9xx13} + 1/{13xx17} + 1/{17xx21} )`

`4D = 3 xx 4 xx ( 1/{1xx5} + 1/{5xx9} + 1/{9xx13} + 4/{13xx17} + 1/{17xx21} )`

`4D = 3 xx ( 4/{1xx5} + 4/{5xx9} + 4/{9xx13} + 4/{13xx17} + 4/{17xx21} )`

`4D = 3 xx ( 1 - 1/5 + 1/5 - 1/9 + 1/9 - 1/13 + 1/13  1/17 + 1/17 - 1/21 )`

`4D = 3 xx ( 1 - 1/21 )`

`4D = 3 xx 20/21`

`4D = 20/7`

`D = 20/7 : 4`

`D = 5/7`

`QH`