show that the points (2,3,4), (-1,-2,1), (5,8,7) are collinear.
Let A (2, 3, 4), B (-1, -2, 1) C (5, 8, 7) Directions of AB = (-1 -2, -2 -3, +1, – 4) =(-3, -5, -3) Dr’s of AC = (5-2, 8-3, 7-4) = (3, 5, 3) Dr’ of AB and AC are proportional. Hence AB and AC are parallel. But A is common ∴ points A, B, C are collinear
