Đáp án:

`C=2/3+2/6+2/12+2/24+...+2/768`

`=2/3 .(1+1/2+1/4+1/8+...+1/256)`

`=>2A=2/3 .( 2+1+1/2+1/4+1/8+...+1/128)`

`=>2A-A=2/3.(2-1/256)`

`=>A=2/3.` $\dfrac{512-1}{256}$

`=>A=511/384`

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