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