If l, m, n are the direction cosines of a line in space, then
- l + m + n = 1
- l² + m² + n² = 1
- lmn = 1
- l² + m² + n² = 0
Show answer and solution
Correct answer: B. l² + m² + n² = 1
1. Direction cosines are the cosines of angles a line makes with the x, y, z axes.
2. If the line has unit direction vector (l, m, n), its squared length equals one.
3. So l² + m² + n² = 1.
4. This is the standard identity for direction cosines.
_Source: syllabus Maths Part 2 Ch 11 "Three Dimensional Geometry", §11.2.1_