Let P (1, –1, 2), Q (3, 4, –2), R (0, 3, 2) and S (3, 5, 6) be given points.
Direction ratios of RS are 3 - 0, 5 - 3, 6 - 2 i.e. 3, 2, 4.
∴ direction cosines of RS are
i.e.,
Projection of PQ on RS
∴ PQ is perpendicular to RS
Hence the result.