What is the value of cos[(180 - θ)/2] cos[(180 - 9θ)/2] + sin[(180 - 3θ)/2] sin[(180 - 13θ)/2] ?
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 ie cos2θ cos6θ.
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= 1/2 [cos4θ - cos5θ + cos8θ + cos5θ]
= 1/2 [cos4θ + cos8θ]
= cos2θ cos6θ