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Find the value of cos (3015°)
1. 1/√2
2. √3/2
3. -1/√2
4. -√3/2 

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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

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