$\text{Bài 4:}$
`a,(2x+3)(4x^2-6x+9)-2(4x^3-1)`
`=8x^3+27-8x^3+2`
`=29`
$\text{Vậy bthuc k phụ thuộc vào x}$
`b,(4x-1)^3-(4x-3)(16x^2+3)`
`=64x^3-48x^2+12x-1-64x^3+48x^2-12x+9`
`=8`
$\text{Vậy bthuc k phụ thuộc vào x}$
`c,2(x^3+y^3)-3(x^2+y^2)` `x+y=1`
`=2(x+y)(x^2-xy+y^2)-3x^2-3y^2`
`=2.1.(x^2-xy+y^2)-3x^2-3y^2`
`=2x^2-2xy+2y^2-3x^2-3y^2`
`=-x^2-2xy-y^2`
`=-(x+y)^2`
`=-1`
`d,(x+1)^3-(x-1)^3-6(x+1)(x-1)`
`=x^3+3x^2+3x+1-x^3+3x^2-3x+1-6x^2+6`
`=8`
$\text{Vậy bthuc k phụ thuộc vào x}$
$\text{Bài 5:}$
`a,(x-1)^3+(2-x)(4+2x+x^2)+3x(x+2)=17`
`⇔x^3-3x^2+3x-1+8-x^3+3x^2+6x=17`
`⇔9x=10`
`⇔x=10/9`
`b,(x+2)(x^2-2x+4)-x(x^2-2)=15`
`⇔x^3+8-x^3+2x=15`
`⇔2x=7`
`⇔x=7/2`
`c,(x-3)^3-(x-3)(x^2+3x+9)+9(x+1)^2=15`
`⇔x^3-9x^2+27x-27-x^3+27+9x^2+18x+9=15`
`⇔45x=6`
`⇔x=2/15`
`d,x(x-5)(x+5)-(x+2)(x^2-2x+4)=3`
`⇔x^3-25x-x^3-8=3`
`⇔-25x=11`
`⇔x=-11/25`
`e,9x^2-6x-3=0`
`⇔3(3x^2-2x-1)=0`
`⇔3x^2-3x+x-1=0`
`⇔3x(x-1)+(x-1)=0`
`⇔(3x+1)(x-1)=0`
`⇔` \(\left[ \begin{array}{l}3x+1=0\\x-1=0\end{array} \right.\) `⇔` \(\left[ \begin{array}{l}x=\dfrac{-1}{3}\\x=1\end{array} \right.\)
`f,x^3+9x^2+27x+19=0`
`⇔x^3+x^2+8x^2+8x+19x+19=0`
`⇔x^2(x+1)+8x(x+1)+19(x+1)=0`
`⇔(x^2+8x+19)(x+1)=0`
`⇔` \(\left[ \begin{array}{l}x^2+8x+19\\x+1=0\end{array} \right.\) `⇔` \(\left[ \begin{array}{l}x∈∅\\x=-1\end{array} \right.\)
