`#Sad`
`a)`
`3x(x-2)-3(x^2-3) = 8`
`⇔ 3x^2-6x-3x^2+9 = 8`
`⇔ -6x = -1`
`⇔ x = 1/6`
`\text{Vậy}` `x=1/6`
`b)`
`2(x-1)(x^2+x+1)-2x(2x^3-x)+x^3(4x-2) = 6`
`⇔ 2(x^3-1)-(4x^4-2x^2)+(4x^4-2x^3) = 6`
`⇔ 2x^3-2-4x^4+2x^2+4x^4-2x^3 = 6`
`⇔ (-4x^4+4x^4)+(2x^3-2x^3)+2x^2-2 = 6`
`⇔ 2x^2 = 8`
`⇔ x^2 = 4`
`⇔` \(\left[ \begin{array}{l}x=2\\x=-2\end{array} \right.\)
`\text{Vậy}` `x=2` `\text{hoặc}` `x=-2`
`\text{→Áp dụng:}` `A^3-B^3 = (A-B)(A^2+AB+B^2)`
`c)`
`(x+2)(x^2-2x+4)-x(x^2-9) = 20`
`⇔ x^3+8-x^3+9x = 20`
`⇔ 9x = 12`
`⇔ x = 4/3`
`\text{Vậy}` `x=4/3`
`\text{→Áp dụng:}` `A^3+B^3 = (A+B)(A^2-AB+B^2)`
