a) `x^2+4x=21`
⇔`x^2+4x-21=0`
⇔`(x^2-3x)+(7x-21)=0`
⇔`x(x-3)+7(x-3)=0`
⇔`(x-3)(x+7)=0`
⇔\(\left[ \begin{array}{l}x-3=0\\x+7=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=3\\x=-7\end{array} \right.\)
Vậy `S={3,-7}`
b) `(x-2)^2-9(x+1)^2=0`
⇔`[x-2-3(x+1)][x-2+3(x+1)]=0`
⇔`(x-2-3x-3)(x-2+3x+3)=0`
⇔`(-2x-5)(4x+1)=0`
⇔\(\left[ \begin{array}{l}-2x-5=0\\4x+1=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=-\dfrac{5}{2}\\x=-\dfrac{1}{4}\end{array} \right.\)
Vậy `S={-5/2,-1/4}`
