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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and .

and  .

Solve these two equations separately.

Consider .

General solution for is and where .

Solutions for in are

and .

where .

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Consider .  

Solutions for in are

and where .  

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Combine above two solutions for complete solution.

  and

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