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Find the value of (cos 25°. cos 35°. cos 85°. sec 75°) × 4
1. 1
2. 2
3. √2
4. √3 
5. √5

1 Answer

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Best answer
Correct Answer - Option 1 : 1

Formula used :

cos θ. cos (60° - θ). cos (60° + θ ) = (1/4) cos 3θ 

sec θ = 1/cos θ 

Calculations :

(cos 25°. cos 35°. cos 85°. sec 75°) × 4 

⇒[cos 25°. cos(60° - 25°) . cos(60°+ 25°). sec 75°] × 4

⇒ [(1/4) cos(3 × 25°). sec 75°] × 4 

⇒ (1/4) [cos 75°. sec 75°] × 4 

⇒ 1

∴ Option 1 is the right answer.

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