Câu 9:
$\vec{AB}(-1-1; -1-3)=(-2;-4)$
$\vec{AC}(3m+7-1; 2m-3-3)=(3m+6; 2m-6)$
Để A, B, C không thẳng hàng:
$\dfrac{3m+6}{-2}\ne \dfrac{2m-6}{-4}$
$\Leftrightarrow 2(3m+6)\ne 2m-6$
$\Leftrightarrow m\ne \dfrac{-9}{2}$
Câu 10:
$M(t; 2t-4)$
$\vec{AB}(3-1; 12-4)=(2;8)$
$\vec{AM}(t-1; 2t-4-4)=(t-1; 2t-8)$
Để A, B, M thẳng hàng:
$\dfrac{t-1}{2}=\dfrac{2t-8}{8}$
$\Leftrightarrow 4(t-1)=2t-8$
$\Leftrightarrow t=-2$
Vậy $M(-2; -8)$
