mn ơi giúp e vs ạ e cảm ơn

`F=1/4 + 1/12 + 1/36 + 1/108 + ... + 1/972`

`=1/4 . (1 + 1/3 + 1/3^2 + ... + 1/3^5) (1)`

Đặt `G=1 + 1/3 + 1/3^2 + ... + 1/3^5`

`=> 3G = 1 + 3 + 1/3 + ... + 1/3^4`

`=> 3G - G = (1 + 3 + 1/3 + ... + 1/3^4)-(1 + 1/3 + 1/3^2 + ... + 1/3^5)`

`=> 2G = 3 - 1/3^5`

`=> G = 3/2 - (1)/(3^5 .2)`

`=> G = 3/2 - 1/486`

`=> G = 364/243 (2)` 

Thay `(2)` vào `(1)`:

`F = 1/4 . 364/243`

`=91/243`.