Giúp em với ạ <3333em kẻm ơn nhìu

`u)` Đặt `U=1/3+1/{3^2}+1/{3^3}+1/{3^4}+...+1/{3^100}`

`3U=1+1/3+1/{3^2}+1/{3^3}+...+1/{3^99}`

`3U-U=(1+1/3+1/{3^2}+1/{3^3}+...+1/{3^99})-(1/3+1/{3^2}+1/{3^3}+1/{3^4}+...+1/{3^100})`

`2U=1-1/{3^100}`

`U={1-1/{3^100}}/2`

`-` Vậy : `U={1-1/{3^100}}/2 .`

`#Sal_tSea`