`B = (1/2^2 - 1)(1/3^2 - 1)(1/4^2 - 1)...(1/2021^2 - 1)`
`⇒ B = (1 - 2^2)/2^2 . (1 - 3^2)/3^2 . (1 - 4^2)/4^2 ... (1 - 2021^2)/2021^2`
`⇒ B = (1 - 4)/(2.2) . (1 - 9)/(3.3) . (1 - 16)/(4.4) ... (1 - 4084441)/(2021.2021)`
`⇒ B = (-3)/(2.2) . (-8)/(3.3) . (-15)/(4.4) ... (-4084440)/(2021.2021)`
`⇒ B = - ( 3/(2.2) . 8/(3.3) . 15/(4.4) ... 4084440/(2021.2021))`
`⇒ B = - ( (1.3)/(2.2) . (2.4)/(3.3) . (3.5)/(4.4) ... (2020.2022)/(2021.2021))`
`⇒ B = - ( (1.2.3...2020)/(2.3.4...2021) . (3.4.5...2022)/(2.3.4...2021))`
`⇒ B = - (1/2021. 2022/2)`
`⇒ B = - (1/2021. 1011)`
`⇒ B = - 1011/2021 < 0 < 1/2`
`⇒ B < 1/2`
`⇒ đpcm`
