a) $3x(25x+15)-35(5x+3)=0$
$⇔3x.5(5x+3)-35(5x+3)=0$
$⇔15x(5x+3)-35(5x+3)=0$
$⇔(5x+3)(15x-35)=0$
$⇔5x+3=0$ hoặc $15x-35=0$
$⇔5x=-3$ hoặc $15x=35$
$⇔x$=$-\frac{3}{5}$ hoặc $x$=$\frac{7}{5}$
b) $(x+2)(3-4x)+(x²+4x+4)=0$
$⇔(x+2)(3-4x)+(x+2)²=0$
$⇔(x+2)(3-4x+x+2)=0$
$⇔(x+2)(-3x+5)=0$
$⇔x+2=$ hoặc $-3x+5=0$
$⇔x=-2$ hoặc $x$=$\frac{5}{3}$
c) $(x-3)(x+4)=(x+4)²$
$⇔(x-3)(x+4)-(x+4)²=0$
$⇔(x+4)(x-3-x-4)=0$
$⇔x+4=0$
$⇔x=-4$