The trigonometric identity on the interval .

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Apply the Sum-to-Product Formula: .

Even-odd property of cosine function: .

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Solve these two equations separately.

Consider .

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General solution for is and where .

Solutions for in  are

and .

where .

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Consider .

General solution for is and where .

Solutions for in are

and where .  

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Combine above two solutions for complete solution.

  and

Arrange in ascending order.

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