Question

If a line makes angles 90°, 135°, 45° with the X-, Y- and Z-axes respectively, then find its direction cosines.

Answer 1

Let l, m, n be the direction cosines of the line.

Then l = cos α, m = cos β, n = cos γ

Here, α = 90°, β = 135°, γ = 45°

∴ l = cos 90° = 0

m = cos 135° = cos (180° - 45°) = - cos 45°

`= - 1/sqrt2` and n = cos 45° = `1/sqrt2`

∴ the direction cosines of the line are 0, `- 1/sqrt2, 1/sqrt2`