Đáp án+Giải thích các bước giải:
a)
$x^3-9x=0$
⇔$x(x^2-9)=0$
⇔$x(x-3)(x+3)=0$
⇔\(\left[ \begin{array}{l}x=0\\x-3=0\\x+3=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=0\\x=3\\x=-3\end{array} \right.\)
Vậy S={0,3,-3}
b)
$2x(x+2)-2x-4=0$
⇔$2x(x+2)-(2x+4)=0$
⇔$2x(x+2)-2(x+2)=0$
⇔$2(x+2)(x-1)=0$
⇔\(\left[ \begin{array}{l}x+2=0\\x-1=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=-2\\x=1\end{array} \right.\)
Vậy S={2,-1}
c)
$x^2+25=10x$
⇔$x^2-10x+25=0$
⇔$x^2-2.5.x+5^2=0$
⇔$(x-5)^2=0$
⇔$x-5=0$
⇔$x=5$
Vậy S={5}
d)
$x^2-5x+4=0$
⇔$(x^2-x)-(4x-4)=0$
⇔$x(x-1)-4(x-1)=0$
⇔$(x-1)(x-4)=0$
⇔\(\left[ \begin{array}{l}x-1=0\\x-4=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=1\\x=4\end{array} \right.\)
Vậy S={1,4}
