Answer: l 2 + m 2 + n 2 = 1.
- A l + m + n = 1
- B lmn = 1
- C l<sup>2</sup> + m<sup>2</sup> + n<sup>2</sup> = 1
- D l<sup>2</sup> + m<sup>2</sup> + n<sup>2</sup> = 0
Correct answer: C. l<sup>2</sup> + m<sup>2</sup> + n<sup>2</sup> = 1
Explanation: The fundamental identity: l<sup>2</sup> + m<sup>2</sup> + n<sup>2</sup> = 1. Direction cosines always satisfy this relation.
Three mutually perpendicular axes x, y, z meeting at the origin O, with a point P located by its (x, y, z) coordinates.
Concept context
Lines and planes in 3D space, direction cosines, distances, and angles