$a)_{}$ $2x^2+3.(x^2-1)=5x.(x+1)_{}$
$⇔2x^2+3x^2-3=5x^2+5x_{}$
$⇔5x^2-3=5x^2+5x_{}$
$⇔-3=5x_{}$
$⇔x=_{}$ $-\frac{3}{5}$
$Vậy_{}$ $x=_{}$ $-\frac{3}{5}$
$b)_{}$ $(x+4)^2-(x+1)(x-1)=16_{}$
$⇔x^2+8x+16-(x^2-1)=16_{}$
$⇔x^2+8x-x^2+1=0_{}$
$⇔8x+1=0_{}$
$⇔8x=-1_{}$
$⇔x_{}=$ $-\frac{1}{8}$
$Vậy_{}$ $x=_{}$ $-\frac{1}{8}$
$c)_{}$ $x.(x+4)-7x-28=0_{}$
$⇔x.(x+4)-7.(x+4)=0_{}$
$⇔(x+4)(x-7)=0_{}$
$⇔_{}$ \(\left[ \begin{array}{l}x+4=0\\x-7=0\end{array} \right.\) $⇔_{}$ \(\left[ \begin{array}{l}x=-4\\x=7\end{array} \right.\)
$Vậy_{}$ $x_{1}=-4$ $;_{}$ $x_{2}=7$
