A (2, –1, 3), B (3, –5, 1) and C (–1, 11, 9).

To Prove: A, B and C are collinear.

Proof:

Let us define position vectors. So,

So, in this case if we prove that and are parallel to each other, then we can easily show that A, B and C are collinear.

Therefore, is given by

And is given by

Let us note the relation between and .

We know,

Or

Or [∵,

This relation shows that and are parallel to each other.

But also, is the common vector in and .

⇒ and are not parallel but lies on a straight line.

Thus, proved that A, B and C are collinear.