$\\$
`B = (1 - 1/4) (1 - 1/9) ... (1 - 1/81) (1 - 1/100)`
`-> B = (4/4 - 1/4) (9/9 - 1/9) ... (81/81 - 1/81) (100/100 - 1/100)`
`->B = 3/4 . 8/9 ... 80/81 . 99/100`
`->B =(1.3)/(2.2) . (2 . 4)/(3.3) ... (8.10)/(9.9) . (9 . 11)/(10.10)`
`-> B = 1/2 . 3/2 . 2/3 . 4/3 ... 8/9 . 10/9 . 9/10 . 11/10`
`->B = (1/2 . 2/3 ... 8/9 . 9/10) . (3/2 . 4/3 .. 10/9 . 11/10)`
`-> B = (1.2...8.9)/(2.3...9.10) . (3.4...10.11)/(2.3...9.10)`
`-> B = (1 . \cancel 2... \cancel 8 . \cancel 9)/(\cancel 2 . \cancel 3 ... \cancel 9 . 10) . (\cancel 3 . \cancel 4 ... \cancel 10 . 11)/(2 . \cancel 3 ... \cancel 9 . \cancel 10)`
`-> B = 1/10 . 11/2`
`-> B = 11/20`
Vì : `20 < 21`
`-> 1/20 > 1/21`
`-> 11/20 > 11/21`
`-> B > 11/21`
Vậy `B > 11/21`