$\"\"$

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The identity is $\"\"$ .

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Consider right hand side identity : $\"\"$ .

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$\"\"$

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Apply the formula : $\"\"$ .

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$\"\"$

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$\"\"$

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Pythagorean identity : $\"\"$ .

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$\"\"$

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$\"\"$

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= Left hand side identity.

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$\"\"$

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$\"\"$ .