`a)`
`1/2x - 1/3x - 1/4 = -1/5`
`=> 1/2x - 1/3x = -1/5 + 1/4`
`=> x . (1/2 - 1/3) = 1/20`
`=> x . 1/6 = 1/20`
`=> x = 1/20 : 1/6`
`=> x = 3/10`
Vậy `x=3/10`
`b)`
`-1/2 - 1/2 (1 - 1/2x) = -1/3`
`=> 1/2 (1-1/2x) = -1/2 + 1/3`
`=> 1/2 (1-1/2x) = -1/6`
`=> 1 - 1/2x = -1/6 : 1/2`
`=> 1 - 1/2x = -1/3`
`=> 1/2x = 1 + 1/3`
`=> 1/2x = 4/3`
`=> x = 4/3 : 1/2`
`=> x = 8/3`
Vậy `x = 8/3`