Lm giúp e 2 vd này vs ah

\begin{array}{c}\color{#003366}{-Gw}\color{#005A9C}{en}\color{#007ACC}{St}\color{#00AEEF}{ac}\color{#00CFFF}{y}\color{#A4D3EE}{-}\color{#D3D3D3}{}\color{#E0E0E0}{}\end{array} 

VD`5`:

`E = 2/3 + 2/9 + 2/27 + 2/81 + 2/243 + 2/729 + 2/2187`

`3E = 3.(2/3 + 2/9 + 2/27 + 2/81 + 2/243 + 2/729 + 2/2187)`

`3E = 2 + 2/3 + 2/9 + 2/27 + 2/81 + 2/243 + 2/729`

`3E - E = (2 + 2/3 + 2/9 + 2/27 + 2/81 + 2/243 + 2/729) - (2/3 + 2/9 + 2/27 + 2/81 + 2/243 + 2/729 + 2/2187)`

`2E = 2 + 2/3 + 2/9 + 2/27 + 2/81 + 2/243 + 2/729 - 2/3 - 2/9 - 2/27 - 2/81 - 2/243 - 2/729 - 2/2187`

`2E = 2 - 2/2187`

`2E = 4372/2187`

`E = 2186/2187`

VD`4` xem lại đề.