Show that points A(-1, 8), B(9, -2) and C(3, 4) are collinear.
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We know that the direction ratios of the line passing through two points P(x1, y1, z1) and Q(x2, y2, z2) are given by:
x2 – x1, y2 – y1, z2 – z1 or x1 – x2, y1 – y2, z1 – z2
Given points are A (2, 3, – 4), B (1, – 2, 3) and C (3, 8, – 11).
Direction ratios of the line joining A and B are:
1 – 2, – 2 – 3, 3 + 4
i.e. – 1, – 5, 7.
The direction ratios of the line joining B and C are:
3 –1, 8 + 2, – 11 – 3
i.e., 2, 10, – 14.
From the above, it is clear that direction ratios of AB and BC are proportional.
That means AB is parallel to BC. But point B is common to both AB and BC.
Hence, A, B, C are collinear points
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