$\"\"$

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The trigonometric identity is $\"\"$ .

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Consider left hand side $\"\"$ .

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$\"\"$

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Apply Pythagorean identity: $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ .

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= Right hand side.

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$\"\"$ .

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$\"\"$

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$\"\"$ .