12/ 3x^2 -x-4=0
<=> (x-4/3)(x+1) = 0
=> Có 2 trường hợp :
x-4/3=0 =>x=4/3
x+1=0   => x=-1
13/ 2x^2 +3x-5 =0
<=> (x-1)(x+5/2) =0
=> Có 2 trường hợp :
x-1=0 =>x=1
x+5/2=0 => x=-5/2

x^2 -5x+6=0 <=> (x-3)(x-2) =0 <=> x=3 hay x=2
x^2 -6x+8 =0 <=> (x-4)(x-2) = 0 <=> x=4 hay x=2

19,
TH1: x +3 = 2x -2  
<=> x = 5 
TH2: - x - 3 = 2x -2
<=> 3x = -1
<=> x = -1/3
20, 
TH1: 3x - (x+1) = 2
<=>  2x = 3
<=>    x =3/2
TH2: 3x + x +1 =2
<=> 4x = 1
<=> x = 1/4
21, 
x - 1 >= 3
<=> x >= 4
22, 
1 - 2x <= 1
<=>  x >= 0 

21/ x-1 >= 3 <=> x>= 4
22/ 1 -2x <= 1 <=> x >= 0

15/x^2-5x+6=0
<=>x^2+x-6x+6=0
<=>x(x+1)-6(x+1)=0
<=>(x+1)(x-6)=0
=>Có 2 trường hợp 
x+1=0 =>x=-1
x-6=0  =>x=6
16/x^2-6x+8=0
<=>x^2-2x-4x+8=0
<=>x(x-2)-4(x-2)=0
<=>(x-2)(x-4)=0
=>Có 2 trường hợp 
x-2=0 =>x=2
x-4=0 =>x=4

18,
TH1: 3x - 2 = 1 
<=> x = 1
TH2: 2 - 3x =1
<=> x = 1/3
23,
3- 1/2*x > 2
<=> 1 > 1/2 *x
<=> x < 2
24,
(x-1)(x+1) < 0
<=> x^2 -1 < 0
<=> x <1
25,
<=> x>1

18/|3x-2|=1
=>Có 2 trường hợp
3x-2=1  =>3x=3  =>x=1
3x-2=-1 =>3x=1  =>x=1/3
19/|x+3|=2x-2
=>Có 2 trường hợp
x+3=2x-2  =>x=5
x+3=-2x+2 =>3x=-1  =>x=-1/3

20, 
TH1: 3x - (x+1) = 2
<=>  2x = 3
<=>    x =3/2
TH2: 3x + x +1 =2
<=> 4x = 1
<=> x = 1/4
21, 
x - 1 ≥ 3
<=> x ≥ 4

22, 
1 - 2x ≤ 1
<=>  x  ≥  0 
23,
3- 1/2*x > 2
<=> 1 > 1/2 *x
<=> x < 2

24,
(x-1)(x+1) < 0
=> x^2 -1 < 0
=> x <1
25,
(x-1)/(3-x) >0
<=> x>1

16) (x^2+x)^2+4(x^2+x)-12=0 *đặt: t=x^2+x *pt =>t^2+4t-12=0 =>t=2 hoặc t=-6 *với t=2 =>x^2+x=2 => x^2+x-2=0 =>x=1 hoặc x=-2 *với t=-6 => x^2+x=-6 => x^2+x+6=0(pt vô n0)

17) (x^2+2x+3)^2-9(x^2+2x+3)^2+18=0 => -8(x^2+2x+3)^2+18=0 => (x^2+2x+3)^2=9/4 => x^2+2x+3=3/2 => x^2+2x+3/2=0 => pt vô n0

12) 3x^2 -x-4=0 <=> (x-4/3)(x+1) = 0 <=> x=4/3 hay x=-1
13) 2x^2 +3x-5 =0 <=> (x-1)(x+5/2) =0 <=> x=1 hay x=-5/2
14) x^2 -5x+6=0 <=> (x-3)(x-2) =0 <=> x=3 hay x=2
15)  x^2 -6x+8 =0 <=> (x-4)(x-2) = 0 <=> x=4 hay x=2
16) 16/x^2-6x+8=0<=>x^2-2x-4x+8=0<=>x(x-2)-4(x-2)=0<=>(x-2)(x-4)=0
+)x-2=0 =>x=2
+)x-4=0 =>x=4
18,TH1: 3x - 2 = 1 <=> x = 1
TH2: 2 - 3x =1<=> x = 1/3
19,
TH1: x +3 = 2x -2  <=> x = 5 
TH2: - x - 3 = 2x -2<=> 3x = -1<=> x = -1/3
20, 
TH1: 3x - (x+1) = 2<=>  2x = 3<=>    x =3/2
TH2: 3x + x +1 =2<=> 4x = 1<=> x = 1/4
21, 
x - 1 >= 3<=> x >= 4
22, 
1 - 2x <= 1<=>  x >= 0 
23,
3- 1/2*x > 2<=> 1 > 1/2 *x<=> x < 2
24,
(x-1)(x+1) < 0<=> x^2 -1 < 0<=> x <1
25,
<=> x>1