$\text {a, 8- x(x+5) =6(3-x) -x²}$
$\text {⇔8-x²-5x =18-6x-x²}$
$\text {⇔x²+5x-6x-x² =8-18}$
$\text {⇔-x =-10}$
$\text {⇔x =10}$
$\text {Vậy S={10}}$
$\text {b, x(2x+3)-10x-15 =0}$
$\text {⇔2x²+3x-10x-15 =0}$
$\text {⇔2x²-7x-15 =0}$
$\text {⇔2x²+3x-10x-15 =0}$
$\text {⇔(2x²+3x)-(10x+15) =0}$
$\text {⇔x(2x+3)-5(2x+3) =0}$
$\text {⇔\(\left[ \begin{array}{l}x-5= 0\\2x+3 =0\end{array} \right.\) }$
$\text {⇔\(\left[ \begin{array}{l}x=5\\2x=-3\end{array} \right.\) }$
$\text {⇔\(\left[ \begin{array}{l}x=5\\x=\frac{-3}{2} \end{array} \right.\) }$
$\text {Vậy S={5; $\frac{-3}{2}$} }$
$\text {Chúc bạn học tốt~}$
