alleryIf l, m, n are the direction cosines of a line in space, then
Al + m + n = 1
Bl² + m² + n² = 1
Clmn = 1
Dl² + m² + n² = 0
Answer & 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_
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