20/5 x 7 + 20/7 x 9 + 20/9 x 11 + 20/11 x 13 +…….+ 20/33 x 35 Cần gấp ạ, đúng và nhanh vote 5 sao + cảm ơn

`20/(5xx7) + 20/(7xx9) + 20/(9xx11) +…+ 20/(33xx35)`

`= 20 xx ( 1/(5xx7) + 1/(7xx9) + 1/(9xx11)+…+ 1/(33xx35))`

`= 10 xx 2( 1/(5xx7) + 1/(7xx9) + 1/(9xx11)+…+ 1/(33xx35))`

`= 10 xx ( 2/(5xx7) + 2/(7xx9) + 2/(9xx11) +…+ 2/(33xx35))`

`= 10 xx  ( 1/5- 1/7 + 1/7 – 1/9+…+ 1/33- 1/35)`

`= 10 xx (1/5- 1/35)`

`= 10 xx 6/35`

`= 12/7`