Bài 18.
a) `(x+2)^2-(x-3)^2-2(x-1)(x+1)=9`
`<=>(x^2+4x+4)-(x^2-6x+9)-2(x^2-1)=9`
`<=>x^2+4x+4-x^2+6x-9-2x^2+2-9=0`
`<=>-2x^2+10x-12=0`
`<=>-2(x^2-5x+6)=0`
`<=>x^2-5x+6=0`
`<=>x^2-2x-3x+6=0`
`<=>x(x-2)-3(x-2)=0`
`<=>(x-2)(x-3)=0`
`<=>`\(\left[ \begin{array}{l}x-2=0\\x-3=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=2\\x=3\end{array} \right.\)
Vậy `S={2;3}`
b) `(x+3)(x^2-3x+9)-x^3=2x`
`⇔x^3+27-x^3-2x=0`
`⇔-2x+27=0`
`⇔-2x=-27`
`⇔x=27/2`
Vậy `S={27/2}`
Bài 20.
a) `978^2+22^2+978.44`
`=978^2+978.22.2+22^2`
`=(978+22)^2`
`=1000^2`
`=1 000 000`
b) `821^2+321^2- 821.642`
`=821^2- 821.2.321+321^2`
`=(821-321)^2`
`=500^2`
`=250 000 `
