a)(x+1)(x+3)-x(x-1)=8
x² +4x+3- x²+x=8
5x=5
x=1
Vậy x=1
b) 9x²= 1-(3x+1)(2x-9)
9x²=1- 6x² +27x -2x +9
15x²-25x-10=0
x²-$\frac{5}{3}$x -$\frac{2}{3}$ =0
x² -2x +$\frac{1}{3}$x -$\frac{2}{3}$=0
x(x-2)+$\frac{1}{3}$(x-2)=0
(x+$\frac{1}{3}$).(x-2)=0
\(\left[ \begin{array}{l}x+\frac{1}{3}\\x-2=0\end{array} \right.\)
\(\left[ \begin{array}{l}x=-\frac{1}{3}\\x=2\end{array} \right.\)
Vậy \(\left[ \begin{array}{l}x=-\frac{1}{3}\\x=2\end{array} \right.\)
