Q.The position vectors of the points $P,Q,R,S$ are $\hat i+\hat j+\hat k,\ 2\hat i+5\hat j,\ 3\hat i+2\hat j-3\hat k$, and $\hat i-6\hat j-\hat k$ respectively. Prove that the line $PQ$ and $RS$ are parallel.

Fix an origin $O$. For any point $P$, the vector $\vec{OP}$ is the position vector of $P$ with respect to $O$. This single vector encodes the point's entire location, and it converts geometry problems into vector algebra.

The fundamental link. For any two points $A,B$ with position vectors $\vec a=\vec{OA}$, $\vec b=\vec{OB}$: $\vec{AB}=\vec{OB}-\vec{OA}=\vec b-\vec a.$

Section formula (internal division). If $P$ divides segment $AB$ internally in the ratio $m:n$ (i.e. $AP:PB=m:n$), then $\vec{OP}=\frac{n\vec a+m\vec b}{n+m}.$ Idea of the proof: since $\vec{AP}$ and $\vec{PB}$ point the same way and $n|\vec{AP}|=m|\vec{PB}|$, we get $n\,\vec{AP}=m\,\vec{PB}$; writing $\vec{AP}=\vec r-\vec a$ and $\vec{PB}=\vec b-\vec r$ (where $\vec r=\vec{OP}$) and solving gives the formula.

Section formula (external division, without proof). If $P$ divides $AB$ externally in the ratio $m:n$: $\vec{OP}=\frac{m\vec b-n\vec a}{m-n}.$

Midpoint. Setting $m=n=1$ in the internal formula: the midpoint of $AB$ has position vector $\dfrac{\vec a+\vec b}{2}$. …