Cho `x= 1/2`
`=> A(1/2) = 1/2 + 1/2^2 + 1/2^3+...+1/2^99+ 1/2^100`
`=> 1/2 A(1/2)= 1/2 ( 1/2 + 1/2^2 + 1/2^3 +...+1/2^99 + 1/2^100)`
`=> 1/2 A(1/2) = 1/2^2 + 1/2^3 +....+1/2^100 + 1/2^101`
`=> A(1/2) - 1/2A (1/2)= 1/2 + 1/2^2 + 1/2^3 +...+1/2^100 - 1/2^2 - 1/2^3 -....-1/2^100 - 1/2^101`
`=> 1/2 A(1/2)= 1/2 - 1/2^101`
`=> A(1/2) = (1/2 - 1/2^101): 1/2`
`=> A(1/2) = (1/2 - 1/2^101) . 2`
`=> A(1/2) = 1 - 1/2^100`
Vậy `A(1/2) = 1- 1/2^100`
