Đáp án+Giải thích các bước giải:
a)
Đặt:
$4x^2$-49=0
⇔$4x^2$-$7^2$=0
⇔(2x-7)(2x+7)=0
⇔\(\left[ \begin{array}{l}2x-7=0\\2x+7=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=\frac{7}{2}\\x=\frac{-7}{2}\end{array} \right.\)
Vậy S={`(7)/(2)`,`(-7)/(2)`}
b)
Đặt:
$x^2$+3x=0
⇔x(x+3)=0
⇔\(\left[ \begin{array}{l}x=0\\x+3=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=0\\x=-3\end{array} \right.\)
Vậy S={-3,0}
c)
Đặt:
$3x^2$+5x-8=0
⇔($3x^2$-3x)+(8x-8)=0
⇔3x(x-1)+8(x-1)=0
⇔(x-1)(3x+8)=0
⇔\(\left[ \begin{array}{l}x-1=0\\3x+8=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=1\\x=\frac{-8}{3}\end{array} \right.\)
Vậy S={1,`(-8)/(3)`}
