helpp me c) S = 1 + $\dfrac{1}{3}$ + $\dfrac{1}{9}$ + $\dfrac{1}{27}$ + $\dfrac{1}{81}$ d) 1 + $\dfrac{1}{ 3}$ + $\dfrac{1}{9}$ + $\dfrac{1}{27}$ +....+ $\dfrac{1}{727}$ + $\dfrac{1}{2181}$

$#Bảo$

Đáp án:

 

Giải thích các bước giải:

`c)`

 `S = 1 +  1/3  +  1/9  +  1/27  +  1/81`

 `S = ( 1/81 + 1/9 ) + ( 1/3 + 1/27 ) + 1`

 `S = ( 1/81 + 9/81 ) + ( 9/27 + 1/27) + 1`

 `S = (10/81 + 10/27) + 1`

 `S = (10/81 + 30/81) + 1`

 `S = 40/81 + 1`

 `S = 40/81 + 1/1`

 `S = 40/81`

Vậy `S = 40/81`

`d)`

`1 + 1/3 + 1/9 + 1/27 + ... + 1/2181`

`3S = 3 + 1 + 1/3 + 1/9 + ... . + 1/727`

`⇒ 3S − S = 3 − 1/2181`

`⇒ 3S = 6542/2181`

`⇒ S = 3271/2181`

Vậy `S = 3271/2181`