Đặt `A = 1 - 1/2 + 1/2^2 - 1/2^3 + 1/2^4 + ... - 1/2^19 + 1/2^20`
`<=> 2A = 2(1 - 1/2 + 1/2^2 - 1/2^3 + 1/2^4 + ... - 1/2^19 + 1/2^20)`
`<=> 2A = 2 - 1 + 1/2 - 1/2^2 + 1/2^3 + ... - 1/2^18 + 1/2^19`
`<=> 2A + A = 2 - 1 + 1/2 - 1/2^2 + 1/2^3 + ... - 1/2^18 + 1/2^19 + 1 - 1/2 + 1/2^2 - 1/2^3 + 1/2^4 + ... - 1/2^19 + 1/2^20`
`<=> 3A = 2 + 1/2^20`
`<=> A = {2 + 1/2^20}/3`