Correct Answer - Option 3 : -1/√2
Concept:
cos (2nπ ± θ) = cos θ
cos (90 + θ) = - sin θ
Calculation:
Given: cos (3015°)
⇒ cos (3015°)
= cos (360° × 8° + 135°)
= cos (135°) [∵ cos (2nπ ± θ) = cos θ ]
= cos (90° + 45°)
= -sin 45° [∵ cos (90 + θ) = - sin θ]
= -1/√2