a)
Th1 : `x - 1 ≥ 0 <=> x ≥ 1`
`x-1 + 3x = 1`
`x = 1/2 (L)`
Th2: `x - 1 < 0 <=> x < 1`
`-x+1 + 3x = 1`
`x = 0 (TM)`
Vậy `x=0`
b)
Th1 : `x - 5 ≥ 0 ; 2x + 1 ≥ 0 <=> x ≥ 5
`x-5 + 2x + 1 =7`
`<=> x = 11/3 (L)`
TH2: `x -5 < 0 ; 2x+1 < 0 <=> x < -1/2`
`-x+5-2x-1 =7`
`x=-1 ( TM)`
Th3 : `x-5 ≥ 0 ; 2x + 1 < 0 <=> x ≥ 5 ; x < -1/2 (VN)`
Th4 : `x - 5 < 0 ; 2x + 1 ≥ 0 <=> x < 5 ; x ≥ -1/2 <=> -1/2 ≤ x < 5`
`-x+5 + 2x+1 = 7`
`x = 1`
