$\\$
Bài `7.`
`A=(1/2 - 1)(1/3-1)...(1/10-1)`
`->A = (1/2-2/2)(1/3-3/3)...(1/10 - 10/10)`
`->A = (-1)/2 . (-2)/3 ... (-9)/10`
`->A = - (1/2 . 2/3 ... 9/10)`
`-> A = (-1)/10`
Vì `10> 9`
`->1/10 < 1/9`
`-> (-1)/10 > (-1)/9`
`->A > (-1)/9`
Vậy `A>(-1)/9`
$\\$
Bài `8.`
`B=(1/4-1)(1/9-1)...(1/100-1)`
`->B=(1/4-4/4)(1/9-9/9)...(1/100 - 100/100)`
`->B=(-3)/4 . (-8)/9 ... (-99)/100`
`-> B = - (3/4 . 8/9 ... 99/100)`
`->B = - ( (1.3)/(2.2) . (2.4)/(3.3) ... (9 .11)/(10.10) )`
`->B = - ( 1/2 . 3/2 . 2/3 . 4/3 ... 9/10 . 11/10)`
`->B = - [(1/2 . 2/3 ... 9/10) . (3/2 . 4/3 ... 11/10) ]`
`->B = - (1/10 . 11/2)`
`->B=(-11)/20`
Vì `20<21`
`->1/20 > 1/21 ->11/20 > 11/21`
`-> (-11)/20 < (-11)/21`
`->B < (-11)/21`
Vây `B<(-11)/21`
