Show that the points `(2,3,4),(-1,-2,1),(5,8,7)` are collinear.

Explanation

Let the points be `A (2,3,4), B (-1,-2,1)` and `C(5,8,7)`.
For line AB direction ratios are,
`-1-2,-2-3,1-4 = - 3, -5, -3`
For line C direction ratios are ,
`5+1,8+2,7-1=6,10,6`
`:'` Direction ratios of AB and BC are proportional
`:.` AB and BC are parallel
But B is a common point in AB and BC.
Therefore A, B and C are collinear.