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Câu `26:`

`3/5 + 2/5 : x=0`

`2/5:x = -3/5`

`x = 2/5 : (-3)/5`

`x = 2/5 xx -5/3`

`x = -2/3`

`=> B`

Câu `27:`

`2/3 : x - 1/2 =1`

`2/3 : x = 1+1/2`

`2/3 : x = 3/2`

`x = 2/3 : 3/2`

`x = 4/9`

`=> B`

Câu `28:`

`(x+3)/2021 + (x+2)/2022 + (x+1)/2023 =-3`

`(x+3)/2021 + (x+2)/2022 + (x+1)/2023 +3=0`

`(x+3)/2021+ 1 + (x+2)/2022 + 1 + (x+1)/2023+1=0`

`(x+3+2021)/2021 + (x+2+2022)/2022 + (x+1+2023)/2023 = 0`

`(x+2024)/2021 + (x+2024)/2022 + (x+2024)/2023=0`

`(x+2024) xx (1/2021 + 1/2022 + 1/2023 ) = 0`

Mà ` (1/2021 + 1/2022 + 1/2023 ) \ne 0` nên `x+2024=0`

`x+2024=0`

`x=-2024`

`=> A`

Câu `29:`

`-8/15 < x/40 < -7/15`

`=> -64/120 < (3x)/120 < -56/120`

`=> -64 < 3x < -56`

`=> 3x in { -63;-62;-61;-60;-59;-58;-57}`

`=> x in { -21;-62/3;-61/3 ;-20;-59/3;-58/3;-19}`

Mà `x in ZZ` nên `x in {-21;-20}`

Do đó có `3` số nguyên `x` thỏa mãn

`=>B`