The trigonometric identity in the interval .

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Apply the Sum-to-Product Formula: .

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Even-odd property of cosine function: .

and .

and .

Solve these two equations separately.

Consider .

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General solution for is and where .

Solutions for in  are and .

and , where ..

The solution in the interval is .

Consider .

Solutions for are and where .  

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The solution in the interval is .

Combine above two solutions for complete solution.

  and

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