alleryFor direction cosines l, m, n of any line in 3D:
AThe simple sum equals one: $l + m + n = 1$
BPythagorean $l^2 + m^2 + n^2 = 1$
CThe simple sum equals zero: $l + m + n = 0$
DThe product equals one: $l × m × n = 1$
Answer & Solution
Correct answer: B. Pythagorean $l^2 + m^2 + n^2 = 1$
**Always:** l² + m² + n² = 1, where l, m, n are cosines of angles with x, y, z axes.
This is the Pythagorean condition for a unit vector.
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