Đáp án:
$\\$
Bài `7`
`a,`
`1/4 + |-1/3 + 7/6| - (-5)/12 + 1/2`
`= 1/4 + |5/6| + 5/12 + 1/2`
`= 1/4 + 5/6 +5/12 + 1/2`
`= (1/4 + 1/2) + (5/6 + 5/12)`
`= 3/4 + 5/4`
`= 2`
$\\$
`b,`
`|-3/4 + 1| - 1/5 + |1/10 - 3/20|`
`= |1/4| - 1/5 + |-1/20|`
`= 1/4 - 1/5 + 1/20`
`= 5/20 - 4/20 + 1/20`
`= 1/10`
$\\$
`c,`
`3/4 - (12/8 - 15/3 + 2/4) - |6/20 - 7/2|`
`= 3/4 - 12/8 + 15/3 - 2/4 - |-16/5|`
`= (3/4 - 12/8 - 2/4) + 15/3 - 16/5`
`= (-3/4 - 2/4) + (15/3 - 16/5)`
`= (-5)/4 + 9/5`
`= (-25)/20 + 36/20`
`= 11/20`
$\\$
`d,`
`|-6/5 + 1/2| - (3/4 - 5/8) + |-3/2|`
`= |-7/10| - 3/4 +5/8 + 3/2`
`= 7/10 - 3/4 + 5/8 + 3/2`
`= (7/10 + 3/2) + (-3/4 + 5/8)`
`= 11/5 - 1/8`
`= 88/40 - 5/40`
`= 83/40`
$\\4
Bài `8`
`A =1/(1×2) + 1/(2×3) + ... + 1/(2019 × 2020)`
`↔ A = 1 - 1/2 + 1/2 - 1/3 + ... + 1/2019 - 1/2020`
`↔ A = 1 + (- 1/2 + 1/2 - 1/3 + ... + 1/2019) - 1/2020`
`↔ A = 1 - 1/2020`
`↔ A = 2020/2020 - 1/2020`
`↔ A = 2019/2020`
Vậy ` = 2019/2020`
