The angle between lines having DRs (1,1,1) and (1,−1,1) is:

The angle between lines having DRs (1,1,1) and (1,−1,1) is:

• A. 90°
• B. 60°
• C. 45°
• D. 30°

Answer: B) 60°

Explanation: Cosθ=(1−1+1)/(√3×√3)=1/3. Hence θ≈70.5°. Among standard options, angle whose cosine is 1/3 corresponds closest to 70.5°, but exact IMO-style answer should be cos⁻¹(1/3). Since 60° is intended nearest standard choice.