$9/_{}$ $2x^2+7x-4=0_{}$
$⇔2x^2+8x-x-4=0_{}$
$⇔2x.(x+4)-(x+4)=0_{}$
$⇔(x+4)(2x-1)=0_{}$
$⇔_{}$ \(\left[ \begin{array}{l}x+4=0\\2x-1=0\end{array} \right.\) $⇔_{}$ \(\left[ \begin{array}{l}x=-4\\x=\frac{1}{2}\end{array} \right.\)
$11/_{}$ $x^3-7x-6=0_{}$
$⇔x^3+x^2-x^2-x-6x-6=0_{}$
$⇔x^2.(x+1)-x.(x+1)-6.(x+1)=0_{}$
$⇔(x+1)(x^2-x-6)=0_{}$
$⇔(x+1)(x^2+2x-3x-6)=0_{}$
$⇔(x+1).[ x.(x+2)-3.(x+2)]=0_{}$
$⇔(x+1)(x+2)(x-3)=0_{}$ ( phần này tự giải ra nhé :D )
$Vậy_{}$ $x_{1}=-1$ $;x_{2}=-2$ $x_{3}=3$
$13/_{}$ $x^3+27+(x+3)(x-9)=0_{}$
$⇔x^3+27+x^2-9x+3x-27=0_{}$
$⇔x^3+x^2-6x=0_{}$
$⇔x.(x^2+x-6)=0_{}$
$⇔x.(x^2+3x-2x-6)=0_{}$
$⇔x.[ x.(x+3)-2.(x+3)]=0_{}$
$⇔x.(x+3)(x-2)=0_{}$ ( đến đây tự làm nhé )
$Vậy_{}$ $x_{1}=0;$ $x_{2}=-3$ $;x_{3}=2$
$15/_{}$ $(2x+1)(4x^2-2x+1)-8x.(x^2+2)=17_{}$
$⇔8x^3+1-8x^3-16x=17_{}$
$⇔1-16x=17_{}$
$⇔-16x=16_{}$
$⇔x_{}=-1$
$Vậy_{}$ $x_{}=-1$
