`B = 1/3 + 1/(3^2) + 1/(3^3) +...+ 1/(3^2019) + 1/(3^2020)`
`3B = 1 + 1/3 + 1/(3^2) + 1/(3^3) +...+ 1/(3^2019)`
`3B - B = 1 - 1/(3^2020) = (3^2020 - 1)/(3^2020)`
`B = (3^2020 - 1)/(3^2020) : 2`
`B = (3^2020 - 1)/(3^2020 . 2)`
`B = 3^2020/(3^2020 . 2) - 1/(3^2020 . 2)`
`B = 1/2 - 1/(3^2020 . 2)`
`⇒ B < 1/2 (đpcm)`