$\"\"$

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The identity is $\"\"$ .

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$\"\"$

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(Apply sum of angles formula $\"\"$ )

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$\"\"$

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(The values of $\"\"$ )

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$\"\"$ $\"\"$

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(Apply sum of angles formula $\"\"$ )

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$\"\"$

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(The values of $\"\"$ )

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$\"\"$

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$\"\"$

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$\"\"$ (Simplify)

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$\"\"$ (Add)

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$\"\"$ (Simplify) $\"\"$

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$\"\"$ is an identity.