Tính nhanh A = 1024-512+256-...-8+4-2+1

$#Harry$

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`A = 1024 - 512 + 256 - ... - 8 + 4 - 2 + 1`

`A = 2^10 - 2^9 + 2^8 - ... - 2^3 + 2^2 - 2^1 + 2^0`

`2A = 2^11 - 2^10 + 2^9 - ... - 2^4 + 2^3 - 2^2 + 2^1`

`2A + A = ( 2^11 - 2^10 + 2^9 - ... - 2^4 + 2^3 - 2^2 + 2^1) + (2^10 - 2^9 + 2^8 - ... - 2^3 + 2^2 - 2^1 + 2^0)`

`3A = 2^11 + 1`

`A = (2^11 + 1)/3`

Vậy `A = (2^11 + 1)/3`