The trigonometric expression is $\"\"$

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Consider the left hand part $\"\"$

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Multiply and divide above expression by 2.

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$\"\"$

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Recall the product to sum formula: $\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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Takeout the common factor.

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$\"\"$

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Hence two expressions are equal.

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Solution:

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$\"\"$

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