3D Geometry

The angle between the lines whose direction cosines satisfy the equations l + m + n = 0 and l2 + m2 + n2 = 0 is

  1. π/2
  2. π/3
  3. π/4
  4. π/6

Answer

l = -m -n

m2 + n2 = (m + n)2

mn = 0

So possibilities are (-1/√2,1/√2,0) and (1/√2,0,1/√2)

The correct option is B.