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`a,(2xxx-5):3+1/4=13/4`
`=> (2xxx-5):3=13/4-1/4`
`=> (2xxx-5):3=3`
`=> 2xxx-5=3xx3`
`=> 2xxx-5=9`
`=> 2xxx=9+5`
`=> 2xxx=14`
`=> x=14:2`
`=> x=7`
Vậy `x=7`
`b, 8/19 - 5/6 xx x = 1/5`
`=> 8/19 - 1/5 = 5/6 xx x`
`=> 5/6 xx x = 21/95`
`=> x = 21/95 : 5/6`
`=> x = 126/475`
Vậy `x=126/475`
`c, 11/12 - (2/3 - 3)xx x = 3/4`
`=> 11/12 - 3/4 = -7/3x`
`=> -7/3x=1/6`
`=> x=1/6:(-7/3)`
`=> x = -1/14`
Vậy `x=-1/14`