$(x +5).( 4x -1) + x^2 -25 =0 $
⇔ $(x+5)(4x-1)+(x-5)(x+5)=0$
⇔ $(x+5)(4x-1+x-5)=0$
⇔ $(x+5)(5x-6)=0$
⇔ \(\left[ \begin{array}{l}x+5=0\\5x-6=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=-5\\5x=6\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=-5\\x=\frac{6}{5}\end{array} \right.\)
Vậy $S=${$-5;\frac{6}{5}$}
$(2x -1).( 3x + 2) – 6.(x+1)^2 =7$
⇔ $6x^2+4x-3x-2-6.(x^2+2x+1)=7$
⇔ $6x^2+4x-3x-2-6x^2-12x-6=7$
⇔ $-11x=7+2+6$
⇔ $-11x=15$
⇔ $x=\frac{-15}{11}$
Vậy $S=${$\frac{-15}{11}$}
