Câu 1.
$x^2-9x+20=0$
$⇔x²-4x-5x+20=0$
$⇔x(x-4)-5(x-4)=0$
$⇔(x-4)(x-5)=0$
$⇔$ \(\left[ \begin{array}{l}x-4=0\\x-5=0\end{array} \right.\)
$⇔$ \(\left[ \begin{array}{l}x=4\\x=5\end{array} \right.\)
Câu 2.
$x^2+2x-15=0$
$⇔x²-3x+5x-15=0$
$⇔x(x-3)+5(x-3)=0$
$⇔(x-3)(x+5)=0$
$⇔$ \(\left[ \begin{array}{l}x-3=0\\x=+5=0\end{array} \right.\)
$⇔$ \(\left[ \begin{array}{l}x=3\\x=-5\end{array} \right.\)
