If $\vec{a}$ and $\vec{b}$ are non-collinear vectors,then the value of $\alpha$ for which the vectors $\vec{u} = (\alpha - 2)\vec{a} + \vec{b}$ and $\vec{v} = (2 + 3\alpha)\vec{a} - 3\vec{b}$ are collinear is:

The position vector of a point that divides the line segment joining $P \equiv(1,2,-1)$ and $Q \equiv(-1,1,1)$ externally in the ratio $1: 2$ is:

If $C$ is the mid-point of the line segment $AB$ and $P$ is any point outside the line $AB$,then

The position vectors of four points $P, Q, R, S$ are $2a + 4c$,$5a + 3\sqrt{3}b + 4c$,$-2\sqrt{3}b + c$,and $2a + c$ respectively. Then:

In a $\triangle ABC$ (shown in the figure below),state whether the following are true or false:
$(i)$ $\vec{AB} + \vec{BC} + \vec{CA} = \vec{0}$
(ii) $\vec{AB} + \vec{BC} - \vec{AC} = \vec{0}$
(iii) $\vec{AB} - \vec{CB} + \vec{CA} = \vec{0}$
(iv) $\vec{AB} + \vec{BC} - \vec{CA} = \vec{0}$

Let $u, v$ and $w$ be non-coplanar vectors. Then the points corresponding to which of the following vectors are collinear?