`A = 1 +2 +4 + 8 +16 + ...+ 8192`
`-> A = 2^0 + 2^1 + 2^2 + 2^3 +2^4 + ... + 2^13`
`-> 2A = 2^1 + 2^2 + 2^3 + 2^4 + 2^5 + ... + 2^14`
`-> 2A - A = (2^1 + 2^2 + 2^3 + 2^4 + 2^5 + ... + 2^14) -(2^0 + 2^1 + 2^2 + 2^3 +2^4 + ... + 2^13)`
`-> A = 2^14 - 2^0 = 2^14 -1`
Vậy `A = 2^14 -1`