Đặt `A = 1 + 1/2 + 1/2^2 + 1/2^4 + 1/2^6 + .... + 1/2^98 + 1/2^100`
`A . 2^2 = 2^2 + 2 + 1 + 1/2^2 + 1/2^4 + .... + 1/2^96 + 1/2^98`
`A . 4 = 4 + 2 + 1 + 1/2^2 + 1/2^4 + .... + 1/2^96 + 1/2^98`
`A . 4 - A = ( 4 + 2 + 1 + 1/2^2 + 1/2^4 + .... + 1/2^96 + 1/2^98 ) - ( 1 + 1/2 + 1/2^2 + 1/2^4 + 1/2^6 + .... + 1/2^98 + 1/2^100 )`
`A . 3 = ( 4 + 2 )- 1/2^100`
`A . 3 = 6 - 1/2^100`
`A = ( 6 - 1/2^100 ) : 3`
`A = ( 6 - 1/2^100 )/3`