Đáp án + giải thích bước giải :
`S = 3 + 3/2 + 3/2^2 + .... + 3/2^{2021}`
`⇔ 1/2S = 3/2 + 3/2^2 + 3/2^3 + ..... + 3/2^{2022}`
`⇔ 1/2S - S = (3/2 + 3/2^2 + 3/2^3 + ..... + 3/2^{2022}) - (3 + 3/2 + 3/2^2 + .... + 3/2^{2021})`
`⇔ (-1)/2 S = 3/2^{2022} - 3`
`⇔ S = (3/2^{2022} - 3)/(-1/2)`