`a, x^2 - (x + 3)(3x + 1) = 9`
`⇔ (x + 3)(x - 3) - (x + 3)(3x + 1) = 0`
`⇔ (x + 3)(-2x - 4) = 0`
`⇔ (x + 3)(x + 2) = 0`
`<=>x=-3` hoặc `x=-2`
`b,x^2+4x+5=0`
`<=>x^2+4x+4-4+5=0`
`<=>(x+2)^2+1=0`
Mà: `(x+2)^2>=0∀x`
`=>` Pt vô no.
`c,(x+14)^2 -(x+12)^2= 1352`
`<=>x^2+28x+196-x^2-24x-144=1352`
`<=>4x+52=1352`
`<=>4x=1300`
`<=>x=325`
`d,x^2+ (x-3)^2= (2x-3)^2`
`<=>2x^2-6x+9=4x^2-12x+9`
`<=>-2x^2+6x=0`
`<=>x(x-3)=0`
`<=>x=0` hoặc `x=3`
`e,x(x+1)(x+2)(x+3)=360`
`<=>x^4+3x^3+2x^3+6x^2+x^3+3x^2+2x^2+6x=360`
`<=>x^4+6x^3+11x^2+6x=360`
`<=>(x^2+3x+20)(x+6)(x-3)=0`
`<=>x=-6` hoặc `x=3`
`f,x^2+ (x-2)(2x+1)=8`
`<=>x^2+2x^2+x-4x-2=8`
`<=>3x^2-3x-2=8`
`<=>3x^2-3x-10=0`
`<=>x=(3+-√129)/6`
Vậy .............................................
