a,
Với $m=1$: $x^2-x-2=0$
$\Delta=1-4.(-2)=9>0$
$\to x_1=\dfrac{1+\sqrt9}{2}=2; x_2=\dfrac{1-\sqrt9}{2}=-1$
b,
$\Delta=(2m-3)^2-4(m^2-3m)$
$=4m^2-12m+9-3m^2+12m=9>0\forall m$
$\to$ PT luôn có hai nghiệm phân biệt:
$x=\dfrac{-2m+3+\sqrt9}{2}=-m+3$
$x=\dfrac{-2m+3-\sqrt9}{2}=-m$
Có $7x_1-8x_2=-1$
$\to \left[ \begin{array}{l}-7m+8(m-3)=-1 \\-7(m-3)+8m=-1\end{array} \right.$
$\to \left[ \begin{array}{l}m=23\\m=-22\end{array} \right.$
