Đáp án:
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`13,`
`-2 (x - 1/2) - 5 (3/10 - 1) = 7`
`↔ -2x + 1 - 3/2 + 5= 7`
`↔ -2x + (1 - 3/2 + 5) = 7`
`↔ -2x + ( (-1)/2 + 5) = 7`
`↔ -2x + 9/2 = 7`
`↔ -2x = 7 - 9/2`
`↔ -2x = 5/2`
`↔ x = 5/2 ÷ (-2)`
`↔ x = 5/2 × (-1/2)`
`↔ x = (-5)/4`
Vậy `x=(-5)/4`
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`14,`
`(-2)/3 (3/2 - x) - (3/4 - x) =3/4 (1/6 - 2/9)`
`↔ -1 + 2/3x - 3/4+ x = 3/4 × (-1)/18`
`↔ (-1 - 3/4) + (2/3x + x) = (-1)/24`
`↔ (-7)/4 + 5/3x = (-1)/24`
`↔ 5/3x = (-1)/24 - (-7)/4`
`↔ 5/3x = 41/24`
`↔ x = 41/24 ÷ 5/3`
`↔ x = 41/24 × 3/5`
`↔ x = 41/40`
Vậy `x=41/40`
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`15,`
`3/4 (x - 8) = 5/7 (14 - 1/2)`
`↔ 3/4x - 6 = 5/7 × 27/2`
`↔ 3/4x - 6 = 135/14`
`↔ 3/4x = 135/14 + 6`
`↔ 3/4x = 219/14`
`↔ x = 219/14 ÷ 3/4`
`↔ x = 219/14 × 4/3`
`↔ x = 146/7`
Vậy `x=146/7`
