`a) x - 1/3 = 1/2`
`x = 1/2 + 1/3`
`x = 5/6`
Vậy `x = 5/6`
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`b) x/21 - 2/3 = 5/21`
`x/21 = 5/21 + 2/3`
`x/21 = 19/21`
⇒ `x = 19`
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`c) 4/5 + 5/7 : x = 1/6`
`5/7 : x = 1/6 - 4/5`
`5/7 : x = (-19/30)`
`x = 5/7 : (-19/30)`
`x = -150/133`
Vậy `x = -150/133`
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`d)(3x + 1)^2 . (-1/4) = -49/4`
`(3x+1)^2 = -49/4 : (-1/4)`
`(3x + 1)^2 = 49`
`(3x + 1)^2 = (+-7)^2`
⇒ `3x + 1 = 7` hoặc `3x + 1 = -7`
`3x = 7 - 1` hoặc `3x = -7 - 1`
`3x = 6` hoặc `3x = -8`
`x = 6 : 3` hoặc `x = -8 : 3`
`x = 2` hoặc `x = -8/3`
Vậy `x ∈ { 2 ; -8/3}`
