If a line makes angles α, β, γ with the coordinate axes, then the value of cos 2α + cos 2β + cos 2γ is:

If a line makes angles α, β, γ with the coordinate axes, then the value of cos 2α + cos 2β + cos 2γ is:

• A. −1
• B. 1
• C. 2
• D. −2

Answer: A) −1

Explanation: cos 2α + cos 2β + cos 2γ = (2cos²α − 1) + (2cos²β − 1) + (2cos²γ − 1) = 2(l² + m² + n²) − 3. Since l² + m² + n² = 1, we get 2(1) − 3 = −1.