`S = 1/2 + 1/2^2 + 1/2^4 + 1/2^6+...+1/2^20 `
`4S = 1/2^2 + 1/2^4 + 1/2^6+1/2^8...+1/2^22`
`4S - S = ( 1/2^2 + 1/2^4 + 1/2^6+1/2^8...+1/2^22) - ( 1/2 + 1/2^2 + 1/2^4 + 1/2^6+...+1/2^20 )`
`3S = 1/2^22 - 1/2 `
mà `1/2^22 < 1 => S < 1`
(Chúc bạn học tốt)