a) `x(x-4)-(x+3)(x-2)=12`
⇔`x^2-4x-x^2+2x-3x+6-12=0`
⇔`-5x-6=0`
⇔`-5x=6`
⇔`x=-6/5`
Vậy `S={-6/5}`
b) `2x(x+3)-2(x+1)(x-5)=-5`
⇔`2x^2+6x-2(x^2-5x+x-5)=-5`
⇔`2x^2+6x-2x^2+10x-2x+10+5=0`
⇔`14x+15=0`
⇔`14x=-15`
⇔`x=-(15)/(14)`
Vậy `S={-(15)/(14)}`
c) `(x-4)(x+2)+(x+3)(4-x)=1`
⇔`x^2+2x-4x-8+4x-x^2+12-3x-1=0`
⇔`-x+3=0`
⇔`-x=-3`
⇔`x=3`
Vậy `S={3}`
